New group call experience: Documentation (#23344)

* Document the use_exclusively Element Call config flag

* Document the new group call experience Labs flag

docs/config.md

@@ -321,6 +321,8 @@ The VoIP and Jitsi options are:
 6. `element_call`: Optional configuration for native group calls using Element Call, with the following subkeys:
    - `url`: The URL of the Element Call instance to use for native group calls. This option is considered experimental
       and may be removed at any time without notice. Defaults to `https://call.element.io`.
+   - `use_exclusively`: A boolean specifying whether Element Call should be used exclusively as the only VoIP stack in
+      the app, removing the ability to start legacy 1:1 calls or Jitsi calls. Defaults to `false`.
 
 ## Bug reporting