Clarify the response body/status of /invite when the user is already invited

[Half-Shot]

Link to problem area: https://spec.matrix.org/v1.2/client-server-api/#post_matrixclientv3roomsroomidinvite

Issue

When a user is invited to the room (via /invite) while already being invited to the room, it's not clear from the spec on /invite what should happen. I couldn't see any text on https://spec.matrix.org/latest/client-server-api/#room-membership on what happens if a user transitions from the same state to the same state either.

Currently, Synapse will just return an empty response body as if you had invited a non-invited user. However, it would be worth clarifying that the endpoint is expected to do that to avoid relying on undefined behaviour.