Document context for why increase timeout for policy server requests (#19633)

See https://github.com/element-hq/synapse/pull/19629#discussion_r3011377886

changelog.d/19633.misc

@@ -0,0 +1 @@
+Document context for why increase timeout for policy server requests.

synapse/handlers/room_policy.py

@@ -223,11 +223,16 @@ class RoomPolicyHandler:
             return
 
         # Ask the policy server to sign this event.
-        # We set a smallish timeout here as we don't want to block event sending too long.
         try:
             signature = await self._federation_client.ask_policy_server_to_sign_event(
                 policy_server.server_name,
                 event,
+                # We set a smallish timeout here as we don't want to block event sending
+                # too long.
+                #
+                # We were previously seeing regular timeouts with media
+                # scanning/checking when the timeout was set to 3s. 30s was chosen based
+                # on vibes and light real world testing.
                 timeout=30000,
             )
             # TODO: We can *probably* remove this when we remove unstable MSC4284 support.