fix broken link

Fixes #6916.

Add a method to the spam checker to filter the user directory results.

Add documentation for the spam checker.

Update documentation to reflect the correct format of user_id (fully qualified).

As using non-C locale can cause issues on upgrading OS.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

* Add note that user_dir requires disabling user dir
  updates from the main synapse process.
* Add note that federation_reader should have
  the federation listener resource.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

This will be used to retry outbound transactions to a remote server if
we think it might have come back up.

Generally try to make this more comprehensible, and make it match the
conventions.

I've removed the documentation for all the settings which allow you to change
the names of the template files, because I can't really see why they are
useful.

Signed-off-by: Manuel Stahl <manuel.stahl@awesome-technologies.de>