[clang-format] Update documentation

- Update `clang-format --help` output after b1f0efc0.
- Update `clang-format-diff.py` help text, which apparently hasn't
  been updated in a while. Since git and svn examples are now part
  of the help text, remove them in the text following the help text.

Differential Revision: https://reviews.llvm.org/D129050