Documentation/locking/atomic: Finish the document...

Documentation/locking/atomic: Finish the document...

Julia reported that the document looked unfinished, and it is. I
forgot to include the example cooked up by Paul here:

  https://lkml.kernel.org/r/20170731174345[email protected]

and I added an explicit example showing how, while it is an ACQUIRE
pattern, it really does provide an MB.

Reported-by: Julia Cartwright <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>