[DOXYGEN] Formatting changes for better intrinsics documentation rendering

(1) I added some \see cross-references to a few select intrinsics that are related (and have the same or similar semantics). 

(2) pmmintrin.h, smmintrin.h, xmmintrin.h have very few minor formatting changes. They make rendering of our intrinsics documentation better. 

llvm-svn: 333065