Matrix: say that the ID is (local) address/alias

Gateway-config-(channel-rules).md

@@ -25,6 +25,7 @@ Below you can find how the channels (`protocolchannel` and `protocol2channel`) s
     - if your team doesn't have multiple channels, use `general`
 - matrix
   - `#channel:server`
+    - (local) address/alias
     - e.g. `#yourchannel:matrix.org`
     - encrypted rooms are not supported in matrix
 - mattermost