Clarify PUT /state event endpoint have special behaviour around `m.room.member`

[ajbura]

Link to problem area:
https://spec.matrix.org/v1.15/client-server-api/#put_matrixclientv3roomsroomidstateeventtypestatekey

Issue
The /state endpoint should clarify that m.room.member state event have special behaviour as per https://spec.matrix.org/v1.15/rooms/v11/#authorization-rules

[tulir]

https://spec.matrix.org/v1.15/client-server-api/#mroommember already describes most of it. Similarly https://spec.matrix.org/v1.15/client-server-api/#mroompower_levels describes the rules for power levels

Also, all calls to PUT /state and PUT /send have to follow auth rules, so technically PUT /state itself doesn't have any special behavior for m.room.member

[ajbura]

For m.room.member it says:

Adjusts the membership state for a user in a room. It is preferable to use the membership APIs (/rooms//invite etc) when performing membership actions rather than adjusting the state directly as there are a restricted set of valid transformations. For example, user A cannot force user B to join a room, and trying to force this state change directly will fail.

it says "there are a restricted set of valid transformations" but doesn't explain them specifically if user want to opt in for direct manipulation.

[tulir]

Might make most sense to add a link like "For precise details on valid transformations, see the authorization rules for the room version" around where it says there are a restricted set