[patch] Improve libstdc++ documentation on using atomics.

[Jonathan Wakely]

Currently we don't mention libatomic anywhere in the libstdc++ manual,
even though it might be needed for std::atomic.

This fixes that and makes a few other drive-by improvements.

Committed to trunk. This would be suitable for all active branches,
so I might backport it once the gcc-5-branch opens.


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