Improve clang-format-diff help output

[thakis]

Summary

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.

[krasimir]

clang/tools/clang-format/clang-format-diff.py

11

Please add a comment that this section serves a dual purpose now.

clang/tools/clang-format/clang-format-diff.py

 #!/usr/bin/env python
 #
 #===- clang-format-diff.py - ClangFormat Diff Reformatter ----*- python -*--===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#
 
-r"""
-ClangFormat Diff Reformatter
-============================
-
+"""
 This script reads input from a unified diff and reformats all the changed
 lines. This is useful to reformat all the lines touched by a specific patch.
 Example usage for git/svn users:
 
   git diff -U0 --no-color HEAD^ | clang-format-diff.py -p1 -i
   svn diff --diff-cmd=diff -x-U0 | clang-format-diff.py -i
 
 """
 from __future__ import absolute_import, division, print_function
 
 import argparse
 import difflib
 import re
 import subprocess
 import sys
 
 if sys.version_info.major >= 3:
     from io import StringIO
 else:
     from io import BytesIO as StringIO
 
 
 def main():
-  parser = argparse.ArgumentParser(description=
-                                   'Reformat changed lines in diff. Without -i '
-                                   'option just output the diff that would be '
-                                   'introduced.')
+  parser = argparse.ArgumentParser(description=__doc__,
+                                   formatter_class=
+                                           argparse.RawDescriptionHelpFormatter)
   parser.add_argument('-i', action='store_true', default=False,
                       help='apply edits to files instead of displaying a diff')
   parser.add_argument('-p', metavar='NUM', default=0,
                       help='strip the smallest prefix containing P slashes')
   parser.add_argument('-regex', metavar='PATTERN', default=None,
                       help='custom pattern selecting file paths to reformat '
                       '(case sensitive, overrides -iregex)')
   parser.add_argument('-iregex', metavar='PATTERN', default=