Improve clang-format-diff help output

The description in clang-format-diff.py is more useful than the one
in `clang-format-diff -h`, so use the same description in both places.

Differential Revision: https://reviews.llvm.org/D64998

llvm-svn: 366828
GitOrigin-RevId: 532f756555717e730166ac882e4c97ac0eeb1664
1 file changed