Make join_rules field of PublicRoomChunk an optional value

proposals/2403-knock.md

@@ -209,15 +209,14 @@ contain this information:
 }
 ```
 
-Therefore this proposal adds `join_rule` as a new, required field to a
-`PublicRoomsChunk`. The `join_rule` of knockable rooms will be `knock`, thus
-giving clients the information they need to attempt entry of
-a room when a client selects it. It also allows clients to display
+Therefore this proposal adds `join_rule` as a new, optional field to a
+`PublicRoomsChunk`. The `join_rule` of knockable rooms will be `knock`,
+thus giving clients the information they need to attempt entry of a
+room when a client selects it. It also allows clients to display
 knockable rooms differently than publicly joinable ones.
 
-For backwards compatibility with old servers, if a client does not find a
-`join_rule` key in a chunk, then it should assume a `join_rule` key with
-value `public`.
+For backwards compatibility with old servers, the default value of
+`join_rule` is `public`.
 
 ### Push Rules