Update clang-modernizer docs

Summary: Docs updated to reflect new behaviour and new options.

Differential Revision: http://llvm-reviews.chandlerc.com/D1841



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@192105 91177308-0d34-0410-b5e6-96231b3b80d8