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
 

docs/labs.md

@@ -168,6 +168,12 @@ Enables support for video rooms that use Element Call rather than Jitsi, and cau
 
 This flag will not have any effect unless `feature_video_rooms` is also enabled.
 
+## New group call experience (`feature_group_calls`) [In Development]
+
+This feature allows users to place and join native [MSC3401](https://github.com/matrix-org/matrix-spec-proposals/pull/3401) group calls in compatible rooms, using Element Call.
+
+If you're enabling this at the deployment level, you may also want to reference the docs for the `element_call` config section.
+
 ## Rich text in room topics (`feature_html_topic`) [In Development]
 
 Enables rendering of MD / HTML in room topics.