Formatting / wording fixes to metrics doc

docs/metrics-howto.rst

 How to monitor Synapse metrics using Prometheus
 ===============================================
 
-1: install prometheus:
+1: Install prometheus:
   Follow instructions at http://prometheus.io/docs/introduction/install/
 
-2: enable synapse metrics:
+2: Enable synapse metrics:
   Simply setting a (local) port number will enable it. Pick a port.
   prometheus itself defaults to 9090, so starting just above that for
   locally monitored services seems reasonable. E.g. 9092:
@@ -15,17 +15,18 @@ How to monitor Synapse metrics using Prometheus
 
   Restart synapse
 
-3: check out synapse-prometheus-config
+3: Check out synapse-prometheus-config
   https://github.com/matrix-org/synapse-prometheus-config
 
-4: arrange for synapse.html to appear in prometheus's "consoles"
-   directory - symlink might be easiest to ensure `git pull` keeps it
-   updated.
+4: Add `synapse.html` and `synapse.rules`
+  The `.html` file needs to appear in prometheus's "consoles" directory, and
+  the `.rules` file needs to be invoked somewhere in the main config file.
+  A symlink to each from the git checkout into the prometheus directory might be
+  easiest to ensure `git pull` keeps it updated.
 
-5: arrange for synapse.rules to be invoked from the main
-   prometheus.conf and add a synapse target. This is easiest if
-   prometheus runs on the same machine as synapse, as it can then just
-   use localhost::
+5: Add a prometheus target for synapse
+  This is easiest if prometheus runs on the same machine as synapse, as it can
+  then just use localhost::
 
     global: {
       rule_file: "synapse.rules"
@@ -39,11 +40,11 @@ How to monitor Synapse metrics using Prometheus
       }
     }
 
-6: start prometheus::
+6: Start prometheus::
 
    ./prometheus -config.file=prometheus.conf
 
-7: wait a few seconds for it to start and perform the first scrape,
+7: Wait a few seconds for it to start and perform the first scrape,
    then visit the console:
 
     http://server-where-prometheus-runs:9090/consoles/synapse.html