Now that we are getting closer to the 7.0.0 release for azure-messaging-servicebus, this issue is to track the work to review all the docs around the package and make any fixes or improvements

• readme
• migration guide ([service bus] Migration guide changes for errors, api changes, etc... #17656)
• java docs
• Adding in sample for how to write a long-running processor ([service bus] Update ServiceBusProcessor sample to demo how to write a long-running processor. #17633) (approved, and in auto-merge)