Disallow invite->knock membership changes

proposals/2403-knock.md

@@ -54,7 +54,7 @@ rules from room version 5 are:
   a. If `membership` is `knock`:
     i. If the `join_rule` is anything other than `knock`, reject.
     ii. If `sender` does not match `state_key`, reject.
-    iii. If the `sender`'s membership is not `ban`, `knock` or `join`, allow.
+    iii. If the `sender`'s membership is not `ban`, `knock`, `invite` or `join`, allow.
     iv. Otherwise, reject.
   ```
 
@@ -67,7 +67,8 @@ rules from room version 5 are:
     - a.ii is justified as one user should not be able to knock on behalf of
       another user.
     - a.iii is justified as knocks should not be allowed if the knocking user
-      has been banned from the room, or if they are already in the room.
+      has been banned from the room, has already knocked on the room, is
+      invited to the room or if they are already in the room.
     - Knocks are not restricted by power level like invites are. The `join_rules`
       are already used to enforce whether someone can or cannot knock.