Improve documentation of workers

Fixes https://github.com/matrix-org/synapse/issues/2554

docs/workers.rst

 Scaling synapse via workers
----------------------------
+===========================
 
 Synapse has experimental support for splitting out functionality into
 multiple separate python processes, helping greatly with scalability.  These
 processes are called 'workers', and are (eventually) intended to scale
 horizontally independently.
 
+All of the below is highly experimental and subject to change as Synapse evolves,
+but documenting it here to help folks needing highly scalable Synapses similar
+to the one running matrix.org!
+
 All processes continue to share the same database instance, and as such, workers
 only work with postgres based synapse deployments (sharing a single sqlite
 across multiple processes is a recipe for disaster, plus you should be using
@@ -16,6 +20,16 @@ TCP protocol called 'replication' - analogous to MySQL or Postgres style
 database replication; feeding a stream of relevant data to the workers so they
 can be kept in sync with the main synapse process and database state.
 
+Configuration
+-------------
+
+To make effective use of the workers, you will need to configure an HTTP
+reverse-proxy such as nginx or haproxy, which will direct incoming requests to
+the correct worker, or to the main synapse instance. Note that this includes
+requests made to the federation port. The caveats regarding running a
+reverse-proxy on the federation port still apply (see
+https://github.com/matrix-org/synapse/blob/master/README.rst#reverse-proxying-the-federation-port).
+
 To enable workers, you need to add a replication listener to the master synapse, e.g.::
 
     listeners:
@@ -27,26 +41,19 @@ Under **no circumstances** should this replication API listener be exposed to th
 public internet; it currently implements no authentication whatsoever and is
 unencrypted.
 
-You then create a set of configs for the various worker processes.  These should be
-worker configuration files should be stored in a dedicated subdirectory, to allow
-synctl to manipulate them.
-
-The current available worker applications are:
- * synapse.app.pusher - handles sending push notifications to sygnal and email
- * synapse.app.synchrotron - handles /sync endpoints.  can scales horizontally through multiple instances.
- * synapse.app.appservice - handles output traffic to Application Services
- * synapse.app.federation_reader - handles receiving federation traffic (including public_rooms API)
- * synapse.app.media_repository - handles the media repository.
- * synapse.app.client_reader - handles client API endpoints like /publicRooms
+You then create a set of configs for the various worker processes.  These
+should be worker configuration files, and should be stored in a dedicated
+subdirectory, to allow synctl to manipulate them.
 
 Each worker configuration file inherits the configuration of the main homeserver
 configuration file.  You can then override configuration specific to that worker,
 e.g. the HTTP listener that it provides (if any); logging configuration; etc.
 You should minimise the number of overrides though to maintain a usable config.
 
-You must specify the type of worker application (worker_app) and the replication
-endpoint that it's talking to on the main synapse process (worker_replication_host
-and worker_replication_port).
+You must specify the type of worker application (``worker_app``). The currently
+available worker applications are listed below. You must also specify the
+replication endpoint that it's talking to on the main synapse process
+(``worker_replication_host`` and ``worker_replication_port``).
 
 For instance::
 
@@ -68,11 +75,11 @@ For instance::
     worker_log_config: /home/matrix/synapse/config/synchrotron_log_config.yaml
 
 ...is a full configuration for a synchrotron worker instance, which will expose a
-plain HTTP /sync endpoint on port 8083 separately from the /sync endpoint provided
+plain HTTP ``/sync`` endpoint on port 8083 separately from the ``/sync`` endpoint provided
 by the main synapse.
 
-Obviously you should configure your loadbalancer to route the /sync endpoint to
-the synchrotron instance(s) in this instance.
+Obviously you should configure your reverse-proxy to route the relevant
+endpoints to the worker (``localhost:8083`` in the above example).
 
 Finally, to actually run your worker-based synapse, you must pass synctl the -a
 commandline option to tell it to operate on all the worker configurations found
@@ -89,6 +96,111 @@ To manipulate a specific worker, you pass the -w option to synctl::
 
     synctl -w $CONFIG/workers/synchrotron.yaml restart
 
-All of the above is highly experimental and subject to change as Synapse evolves,
-but documenting it here to help folks needing highly scalable Synapses similar
-to the one running matrix.org!
+
+Available worker applications
+-----------------------------
+
+``synapse.app.pusher``
+~~~~~~~~~~~~~~~~~~~~~~
+
+Handles sending push notifications to sygnal and email. Doesn't handle any
+REST endpoints itself, but you should set ``start_pushers: False`` in the
+shared configuration file to stop the main synapse sending these notifications.
+
+Note this worker cannot be load-balanced: only one instance should be active.
+
+``synapse.app.synchrotron``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The synchrotron handles ``sync`` requests from clients. In particular, it can
+handle REST endpoints matching the following regular expressions::
+
+    ^/_matrix/client/(v2_alpha|r0)/sync$
+    ^/_matrix/client/(api/v1|v2_alpha|r0)/events$
+    ^/_matrix/client/(api/v1|r0)/initialSync$
+    ^/_matrix/client/(api/v1|r0)/rooms/[^/]+/initialSync$
+
+The above endpoints should all be routed to the synchrotron worker by the
+reverse-proxy configuration.
+
+It is possible to run multiple instances of the synchrotron to scale
+horizontally. In this case the reverse-proxy should be configured to
+load-balance across the instances, though it will be more efficient if all
+requests from a particular user are routed to a single instance. Extracting
+a userid from the access token is currently left as an exercise for the reader.
+
+``synapse.app.appservice``
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Handles sending output traffic to Application Services. Doesn't handle any
+REST endpoints itself, but you should set ``notify_appservices: False`` in the
+shared configuration file to stop the main synapse sending these notifications.
+
+Note this worker cannot be load-balanced: only one instance should be active.
+
+``synapse.app.federation_reader``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Handles a subset of federation endpoints. In particular, it can handle REST
+endpoints matching the following regular expressions::
+
+    ^/_matrix/federation/v1/event/
+    ^/_matrix/federation/v1/state/
+    ^/_matrix/federation/v1/state_ids/
+    ^/_matrix/federation/v1/backfill/
+    ^/_matrix/federation/v1/get_missing_events/
+    ^/_matrix/federation/v1/publicRooms
+
+The above endpoints should all be routed to the federation_reader worker by the
+reverse-proxy configuration.
+
+``synapse.app.federation_sender``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Handles sending federation traffic to other servers. Doesn't handle any
+REST endpoints itself, but you should set ``send_federation: False`` in the
+shared configuration file to stop the main synapse sending this traffic.
+
+Note this worker cannot be load-balanced: only one instance should be active.
+
+``synapse.app.media_repository``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Handles the media repository. It can handle all endpoints starting with::
+
+    /_matrix/media/
+
+Note this worker cannot be load-balanced: only one instance should be active.
+
+``synapse.app.client_reader``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Handles client API endpoints. It can handle REST endpoints matching the
+following regular expressions::
+
+    ^/_matrix/client/(api/v1|r0|unstable)/publicRooms$
+
+``synapse.app.user_dir``
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Handles searches in the user directory. It can handle REST endpoints matching
+the following regular expressions::
+
+    ^/_matrix/client/(api/v1|r0|unstable)/user_directory/search$
+
+``synapse.app.frontend_proxy``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Proxies some frequently-requested client endpoints to add caching and remove
+load from the main synapse. It can handle REST endpoints matching the following
+regular expressions::
+
+    ^/_matrix/client/(api/v1|r0|unstable)/keys/upload
+
+It will proxy any requests it cannot handle to the main synapse instance. It
+must therefore be configured with the location of the main instance, via
+the ``worker_main_http_uri`` setting in the frontend_proxy worker configuration
+file. For example::
+
+    worker_main_http_uri: http://127.0.0.1:8008
+