Fix join membership auth rules when join_rule is knock (#3737)

Fixes #3736

Author: tulir

changelogs/room_versions/newsfragments/3737.clarification

@@ -0,0 +1 @@
+Fix join membership auth rules when `join_rule` is `knock`.

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

@@ -53,8 +53,8 @@ The rules are as follows:
             `state_key` is the creator, allow.
         2.  If the `sender` does not match `state_key`, reject.
         3.  If the `sender` is banned, reject.
-        4.  If the `join_rule` is `invite` then allow if membership
-            state is `invite` or `join`.
+        4.  If the `join_rule` is `invite` or `knock` then allow if
+            membership state is `invite` or `join`.
         5.  If the `join_rule` is `restricted`:
             1.  If membership state is `join` or `invite`, allow.
             2.  If the `join_authorised_via_users_server` key in `content`

content/rooms/v7.md

@@ -82,8 +82,8 @@ The rules are as follows:
             `state_key` is the creator, allow.
         2.  If the `sender` does not match `state_key`, reject.
         3.  If the `sender` is banned, reject.
-        4.  If the `join_rule` is `invite` then allow if membership
-            state is `invite` or `join`.
+        4.  If the `join_rule` is `invite` or `knock` then allow if
+            membership state is `invite` or `join`.
         5.  If the `join_rule` is `public`, allow.
         6.  Otherwise, reject.
     3.  If `membership` is `invite`: