[DOXYGEN] Fix doxygen and content issues in avxintrin.h

- Fix incorrect wording in various intrinsic descriptions. Previously the descriptions used "low-order" and "high-order" when the intended meaning was "even-indexed" and "odd-indexed".
- Fix a few typos and errors found during review.
- Restore new line endings.

This patch was made by Craig Flores

llvm-svn: 322027