[clang-tidy] correct clang-tidy-diff.py help message

Summary:
Looks like the original code was copied from clang-format-diff.py.
Update help message to make it clang-tidy specific.

Reviewers: klimek, alexfh

Subscribers: Eugene.Zelenko, cfe-commits

Patch by Igor Sugak!

Differential Revision: http://reviews.llvm.org/D21050

llvm-svn: 272144

clang-tools-extra/clang-tidy/tool/clang-tidy-diff.py

@@ -33,20 +33,20 @@
 
 def main():
   parser = argparse.ArgumentParser(description=
-                                   'Reformat changed lines in diff. Without -i '
-                                   'option just output the diff that would be '
-                                   'introduced.')
+                                   'Run clang-tidy against changed files, and '
+                                   'output diagnostics only for modified '
+                                   'lines.')
   parser.add_argument('-clang-tidy-binary', metavar='PATH',
                       default='clang-tidy',
                       help='path to clang-tidy binary')
   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 '
+                      help='custom pattern selecting file paths to check '
                       '(case sensitive, overrides -iregex)')
   parser.add_argument('-iregex', metavar='PATTERN', default=
                       r'.*\.(cpp|cc|c\+\+|cxx|c|cl|h|hpp|m|mm|inc)',
-                      help='custom pattern selecting file paths to reformat '
+                      help='custom pattern selecting file paths to check '
                       '(case insensitive, overridden by -regex)')
 
   parser.add_argument('-fix', action='store_true', default=False,