Here is the patch with the doxygen comments for the intrinsincs in the header file popcntintrin.h.
The doxygen comments are automatically generated based on SCE internal intrinsics document using DCG tool that I wrote.
I will submit more doxygen comments for the other intrinsic header files as soon as this patch is approved.
Here is the link to the general discussion about adding comments to x86 intrinsics headers.
Here are the links to the similar code reviews for the doxygen comments in some other header files.