notifications: Document an example scenario where builds_finished is triggered twice.

Dependencies

• [2] 6A5QEB4Q Write up some documentation on notifications

Change contents

• edit in doc/manual/src/notifications.md at line 36
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  + It is possible for subsequent deliveries of the same `build_finished` data to imply different outcomes. For example, if the build fails, is restarted, and then succeeds. In this scenario the `build_finished` events will be delivered at least twice, once for the failure and then once for the success.