Doc rendering is exactly as before, but it now actually tests the code

Prevents the server from crashing/become unresponsive when overly long
messages are sent

Fix the scenario where a MembershipState change event was not sent to the server of a user kicked/banned from a room on a Conduit instance if there were not any other users from that server in the room.

Fixes #110.

To my understanding, a bad user ID can sometimes make it into the
database, which lead to a panic prior to this change.