Marking from parameter as optional

As per MSC3567, the `from` parameter is now optional for the `/messages` endpoint to allow fetching first or latest room content without having to rely on `/sync`

#3567

Author: germain-gg

data/api/client-server/message_pagination.yaml

@@ -57,7 +57,11 @@ paths:
             This endpoint can also accept a value returned as a `start` token
             by a previous request to this endpoint, though servers are not
             required to support this. Clients should not rely on the behaviour.
-          required: true
+
+            If it is not provided, the homeserver shall return a list of messages 
+            from the first or last (per the value of the `dir` parameter) visible
+            event in the room history for the requesting user.
+          required: false
           x-example: "s345_678_333"
         - in: query
           type: string