diff --git a/clang/tools/clang-format/clang-format-diff.py b/clang/tools/clang-format/clang-format-diff.py
--- a/clang/tools/clang-format/clang-format-diff.py
+++ b/clang/tools/clang-format/clang-format-diff.py
@@ -13,7 +13,7 @@
 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
+  git diff -U0 --no-color --relative HEAD^ | clang-format-diff.py -p1 -i
   svn diff --diff-cmd=diff -x-U0 | clang-format-diff.py -i
 
 """
@@ -36,7 +36,9 @@
                                    formatter_class=
                                            argparse.RawDescriptionHelpFormatter)
   parser.add_argument('-i', action='store_true', default=False,
-                      help='apply edits to files instead of displaying a diff')
+                      help='apply edits to files instead of displaying a diff.'
+                      ' The filename contained in the diff is used unmodified'
+                      ' to determine the source file to update')
   parser.add_argument('-p', metavar='NUM', default=0,
                       help='strip the smallest prefix containing P slashes')
   parser.add_argument('-regex', metavar='PATTERN', default=None,