Reformat clang-format help strings, filter out irrelevant options.

Summary: +updated ClangFormat.rst

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D780

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181617 91177308-0d34-0410-b5e6-96231b3b80d8