Document and use AtomicStack concurrency properties

Document the fact that most AtomicStack operations are not atomic,
and document the one case in which they are.

I initially believed, based on its implementation, that PopBackCount
was also intended to be callable concurrently with other PopBackCount
calls. But the SequentiallyConsistent requirement still made
no sense, since there can't safely be any concurrent readers.
On closer inspection, its only client assumes that it has exclusive
access to the stack. Thus this removes the unused, and probably
unusable, thread-safety overhead.

Test: Builds. TreeHugger.

Bug: 62270718
Change-Id: I38e8413fd1851af59a30313f28864496f1d9458b

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