Mention that knock/private on join_rules are not implemented.

event-schemas/schema/v1/m.room.join_rules

@@ -2,7 +2,7 @@
     "$schema": "http://json-schema.org/draft-04/schema#",
     "type": "object",
     "title": "Describes how users are allowed to join the room.",
-    "description": "A room may be ``public`` meaning anyone can join the room without any prior action. Alternatively, it can be ``invite`` meaning that a user who wishes to join the room must first receive an invite to the room from someone already inside of the room.",
+    "description": "A room may be ``public`` meaning anyone can join the room without any prior action. Alternatively, it can be ``invite`` meaning that a user who wishes to join the room must first receive an invite to the room from someone already inside of the room. Currently, ``knock`` and ``private`` are reserved keywords which are not implemented.",
     "allOf": [{
         "$ref": "core#/definitions/state_event"
     }],