[DOXYGEN] Improved doxygen comments for avxintrin.h intrinsics.

Tagged parameter names with \a doxygen command to display them in italics.
Formatted comments to fit into 80 chars.




git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289022 91177308-0d34-0410-b5e6-96231b3b80d8