[skip ci] ActionCable notable changes added to 7.1 release note

Author: akhilgkrishnan

guides/source/7_1_release_notes.md

@@ -197,6 +197,12 @@ Please refer to the [Changelog][action-cable] for detailed changes.
 
 ### Notable changes
 
+*   Add `capture_broadcasts` test helper to capture all messages broadcasted in a block.
+
+*   Add the ability to Redis pub/sub adapter to automatically reconnect when Redis connection is lost.
+
+*   Add command callbacks `before_command`, `after_command`, and `around_command` to `ActionCable::Connection::Base`.
+
 Action Pack
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