Add feature_disable_call_per_sender_encryption documentation (in labs…

….md) (#26548)

* add feature_disable_call_per_sender_encryption doc

Co-authored-by: aceArt-GmbH <33117017+aceArt-GmbH@users.noreply.github.com>

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Signed-off-by: Timo K <toger5@hotmail.de>
Co-authored-by: aceArt-GmbH <33117017+aceArt-GmbH@users.noreply.github.com>

docs/labs.md

@@ -120,6 +120,15 @@ This feature allows users to place and join native [MSC3401](https://github.com/
 
 If you're enabling this at the deployment level, you may also want to reference the docs for the `element_call` config section.
 
+## Disable per-sender encryption for Element Call (`feature_disable_call_per_sender_encryption`)
+
+The default for embedded Element Call in Element Web is per-participant encryption.
+This labs flag disables encryption for embedded Element Call in encrypted rooms.
+
+Under the hood this stops Element Web from adding the `perParticipantE2EE` flag for the Element Call widget url.
+
+This is useful while we experiment with encryption and to make calling compatible with platforms that don't use encryption yet.
+
 ## Rich text in room topics (`feature_html_topic`) [In Development]
 
 Enables rendering of MD / HTML in room topics.