doc: change backporting guide with updated info
This PR updates the information in the backporting guide to (A) follow the Microsoft Style Guide, and (B) provide clearer instructions on how to backport.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
This PR updates the information in the backporting guide to (A) follow the Microsoft Style Guide, and (B) provide clearer instructions on how to backport.