Added documentation for clang-format style options.

Summary:
The main contents is in the ClangFormatStyleOptions.rst, which can be
updated from the Format.h by the dump_format_style.py script.

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1597

llvm-svn: 189946