HaikuBook: Initial documentation for BNotification

The implementation file contained some documentation. This has been moved
to the Haiku Book (and is rewritten in most cases). The documentation gives
some insight on how the notification_server works.

Change-Id: I82bafcf57101d4882bdf07e7f731df9cd8adc861
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2299
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>