Expand explanation of atomicity.

Mention per-location total order, out of thin air, and torn writes
guarantees. Mention C11 standard' memory model and one most important
FreeBSD additional requirement, that is aligned ordinary loads and
stores are atomic on processors.

The text is introductional and informal. Reference the C11 and
C++1{1,4,7} standards for authorative description.

In collaboration with: alc
Sponsored by: The FreeBSD Foundation (kib)
MFC after: 1 week