Document and warn that invoking `/remove` to self actually parts you from the room

### Your use case

Per Matrix spec, kicking is just a forced leave issued to another user by a user with a sufficient power level. People without kicking powers can still use `/remove` against themselves, as this case is unchecked in the client side (this is totally OK server-side, as explained in the first sentence)

### Have you considered any alternatives?

Simply not send the leave event when the operand of the slash command `/remove` is yourself.

### Additional context

_No response_