Modify event auth rules not to rely on depth

Signed-off-by: Marcus Medom Ryding <mrok4a@gmail.com>

specification/server_server_api.rst

@@ -504,15 +504,15 @@ the state of the room.
           state of the room. For example, a redacted ``join`` event will still
           result in the user being considered joined.
 
-1. If type is ``m.room.create``, allow if and only if has depth 0 and it has no
+1. If type is ``m.room.create``, allow if and only if it has no
    previous events - *i.e.* it is the first event in the room.
 
 #. If type is ``m.room.member``:
 
    a. If ``membership`` is ``join``:
 
-      i. If the previous event in the room graph is an ``m.room.create``, the
-         depth is 1 and the ``state_key`` is the creator, allow.
+      i. If the only previous event is an ``m.room.create``
+         and the ``state_key`` is the creator, allow.
 
       #. If the ``sender`` does not match ``state_key``, reject.