Add some explanation to application_services.md (matrix-org#7091)

Signed-off-by: Simon Körner <git@lubiland.de>

Author: lub

changelog.d/7091.doc

@@ -0,0 +1 @@
+Improve the documentation of application service configuration files.

docs/application_services.md

@@ -23,9 +23,13 @@ namespaces:
   users:  # List of users we're interested in
     - exclusive: <bool>
       regex: <regex>
+      group_id: <group>
     - ...
   aliases: []  # List of aliases we're interested in
   rooms: [] # List of room ids we're interested in
 ```
 
+`exclusive`: If enabled, only this application service is allowed to register users in its namespace(s).
+`group_id`: All users of this application service are dynamically joined to this group. This is useful for e.g user organisation or flairs.
+
 See the [spec](https://matrix.org/docs/spec/application_service/unstable.html) for further details on how application services work.