Rollup merge of #147887 - matthieu-m:task/lib-core-sync-atomic-fence-doc-improvement, r=Mark-Simulacrum

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.

Author: matthiaskrgr