Update cross-signing feature docs and document fallback procedures

For https://github.com/matrix-org/matrix-react-sdk/pull/4416

docs/labs.md

@@ -67,13 +67,24 @@ An implementation of [MSC2241](https://github.com/matrix-org/matrix-doc/pull/224
 
 This also includes a new implementation of the user & member info panel, designed to share more code between showing community members & room members. Built on top of this new panel is also a new UX for verification from the member panel.
 
-## Cross-signing (in development) (`feature_cross_signing`)
+## Cross-signing
 
 Cross-signing ([MSC1756](https://github.com/matrix-org/matrix-doc/pull/1756))
 improves the device verification experience by allowing you to verify a user
 instead of verifying each of their devices.
 
-This feature is still in development and will be landing in several chunks.
+The feature is enabled by default and does not follow a traditional labs flag
+at the moment. If something goes wrong, add this to your config to disable it:
+```json
+{
+  "settingDefaults": {
+    "feature_cross_signing": false  
+  }
+}
+```
+
+The setting will be removed in a future release, enabling it non-optionally for
+all users.
 
 ## Event indexing and E2EE search support using Seshat (`feature_event_indexing`)