Doxygen comments for avxintrin.h.

Added doxygen comments to avxintrin.h's intrinsics. As of now, only around 50% of the intrinsics in this file are documented here. The patches for the other half will be sent out later.

Updated bmiintrin.h to fix an incorrect section name.
Updated f16cintrin.h to fix incorect parameter names.


The doxygen comments are automatically generated based on Sony's intrinsics document.

I got an OK from Eric Christopher to commit doxygen comments without prior code
review upstream.

llvm-svn: 269718