Improve the documentation of atomic::fence Attempt to "fix" two flaws of the current documentation: 1. The over-emphasis of fence - fence synchronization, relegating atomic - fence and fence - atomic synchronization to second fiddle. 2. The lack of explanation as to how to properly perform atomic - fence and fence - atomic synchronization. It does so by first making it clear that there are 3 different ways to use an atomic fence, then presenting a full example for each usecase, noting the particular position of the fence with regard to the atomic operation, and rounding up with generic notes.