Attempt to define a valid event (#1045)

* Attempt to define a valid event

Resolves #1044.

Well, maybe.

* Changelog

* link to /rooms, thanks Travis

Co-authored-by: Travis Ralston <travpc@gmail.com>

Co-authored-by: Travis Ralston <travpc@gmail.com>

changelogs/server_server/newsfragments/1045.clarification

@@ -0,0 +1 @@
+Expand a little on what it means for a PDU to be valid when discussing checks on PDUs.

content/server-server-api.md

@@ -356,7 +356,9 @@ specification](/rooms).
 Whenever a server receives an event from a remote server, the receiving
 server must ensure that the event:
 
-1.  Is a valid event, otherwise it is dropped.
+1.  Is a valid event, otherwise it is dropped. For an event to be valid, it
+    must contain a `room_id`, and it must comply with the event format of
+    that [room version](/rooms).
 2.  Passes signature checks, otherwise it is dropped.
 3.  Passes hash checks, otherwise it is redacted before being processed
     further.