[v3] api docs

[Benjamin Kosnik]

Adds some more bits. I'm not sure how to best approach backwards
compat. This gets some ideas out, although this needs further
refinement. Ideally autoconf macros can migrate to the autoconf macro
repository.

Some points have to be resolved.

1) cxxabi-forced.h needs some kind of description. Should this just be
in cxxabi.h?

2) _GLIBCXX_ATOMIC_BUILTINS versus __default_lock_policy's
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2

2) _Atomic_word in global namespace or __gnu_cxx?

tested x86_64/linux

-benjamin
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