Fix knock->leave transition in auth rules (#3694)

* Fix knock->leave transition in auth rules

This was an oversight from knocking being added.

For safety, this has been verified as at least intended by Synapse to work:
f5e2cde3f5/synapse/event_auth.py (L390-L391)

* changelog

changelogs/room_versions/newsfragments/3694.clarification

@@ -0,0 +1 @@
+Fix auth rules to allow membership of `knock` -> `leave` in v7, v8, and v9.

content/rooms/fragments/v8-auth-rules.md

@@ -91,7 +91,8 @@ The rules are as follows:
         5.  Otherwise, reject.
     5.  If `membership` is `leave`:
         1.  If the `sender` matches `state_key`, allow if and only if
-            that user's current membership state is `invite` or `join`.
+            that user's current membership state is `invite`, `join`,
+            or `knock`.
         2.  If the `sender`'s current membership state is not `join`,
             reject.
         3.  If the *target user*'s current membership state is `ban`,

content/rooms/v7.md

@@ -114,7 +114,8 @@ The rules are as follows:
         5.  Otherwise, reject.
     4.  If `membership` is `leave`:
         1.  If the `sender` matches `state_key`, allow if and only if
-            that user's current membership state is `invite` or `join`.
+            that user's current membership state is `invite`, `join`,
+            or `knock`.
         2.  If the `sender`'s current membership state is not `join`,
             reject.
         3.  If the *target user*'s current membership state is `ban`,