Improve the documentation of the optimization flags

Reviewers: rafael.espindola, rengolin, hfinkel

Reviewed By: rengolin

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2138

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194405 91177308-0d34-0410-b5e6-96231b3b80d8