Clarify that age is generated on the local HS

Signed-off-by: Johannes Löthberg <johannes@kyriasis.com>

event-schemas/schema/core-event-schema/room_event.yaml

@@ -22,7 +22,9 @@ properties:
     properties:
       age:
         description: The time in milliseconds that has elapsed since the event was
-          sent
+          sent. This field is generated by the local homeserver, and may be incorrect
+          if the local time on at least one of the two servers is out of sync, which can
+          cause the age to either be negative or greater than it actually is.
         type: integer
       redacted_because:
         description: Optional. The event that redacted this event, if any.