diagnostics_channel: built-in channels should remain experimental

PR-URL: #45423
Reviewed-By: theanarkh <theratliter@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>

doc/api/diagnostics_channel.md

@@ -403,6 +403,12 @@ channel.unsubscribe(onMessage);
 
 ### Built-in Channels
 
+> Stability: 1 - Experimental
+
+While the diagnostics\_channel API is now considered stable, the built-in
+channels currently available are not. Each channel must be declared stable
+independently.
+
 #### HTTP
 
 `http.client.request.start`