Description of QR code sign in labs feature (#23513)

Author: hughns

docs/labs.md

@@ -185,3 +185,7 @@ have [MSC3827](https://github.com/matrix-org/matrix-spec-proposals/pull/3827) en
 ## Favourite Messages (`feature_favourite_messages`) [In Development]
 
 Enables users to bookmark a message or content for a later reference.
+
+## Sign in another device by showing a QR code (`feature_qr_signin_reciprocate_show`)
+
+Add capability to the session/device manager screens to generate a QR code to sign in another device + set up E2EE. This requires the homeserver to have support for [MSC3882](https://github.com/matrix-org/matrix-spec-proposals/pull/3882) and [MSC3886](https://github.com/matrix-org/matrix-spec-proposals/pull/3886) enabled.