Enumerate all events created when a room is created (SPEC-409)

[matrixbot]

In section 7.1 (Rooms: Creation) of the client-server spec, it's stated that creating a room generates several events, and lists m.room.create, m.room.power_levels, and m.room.join_rules. Reading through the Synapse code, there are many more events that may be created. The spec should enumerate all the events that might be created along with a new room, and if it's not obvious, how their values are decided and in which cases they aren't created at all.

(Imported from https://matrix.org/jira/browse/SPEC-409)

(Reported by Jimmy Cuadra)