Account data section needs cleanup

[richvdh]

Link to problem area:

https://spec.matrix.org/v1.12/client-server-api/#client-config

Issue

• For some reason, it is called "Client config" in the spec, when literally nobody calls it that
• It refers to account data as "events", when they really really aren't that. (They might appear as event-like things when sent down /sync or /events, but that's something else.) Related: Spec refers to EDUs and account data as "events" #897
• Lots of bits of the spec refer to "account data" but don't hyperlink back to the spec section on account data
• Room tags are built on top of account data, but appear before account data in the spec, so that feels backwards.
• The "client config" (i.e. account data) section has specific information about m.tag ("m.tag events appearing in /events will have a room_id with the room the tags are for"), which seems like it should be common to all room-based account data.

[richvdh]

I've labelled this as "good first issue", but if you'd like to pick this up as the first issue you work on, I suggest starting by fixing one of the things in the above list, rather than trying to solve them all at once.