[DOXYGEN] Minor improvements in doxygen comments.

- To be consistent with the rest of the intrinsics headers, I removed the tags <i> .. </i> for marking instruction names in italics in in smmintrin.h. 

- Formatting changes to fit into 80 characters. 

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

llvm-svn: 300578