Document the maximum value for depth

Implements the proposal for https://github.com/matrix-org/matrix-doc/issues/1230

api/server-server/definitions/unsigned_pdu.yaml

@@ -78,7 +78,10 @@ properties:
           required: ['sha256']
   depth:
     type: integer
-    description: The maximum depth of the ``prev_events``, plus one.
+    description: |-
+      The maximum depth of the ``prev_events``, plus one. Must be less than the
+      maximum value for an integer (2^63 - 1). If the room's depth is already at
+      the limit, the depth must be set to the limit.
     example: 12
   auth_events:
     type: array