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
@@ -13,9 +13,13 @@
 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
 
+It should be noted that the filename contained in the diff is used unmodified
+to determine the source file to update. Users calling this script directly
+should be careful to ensure that the path in the diff is correct relative to the
+current working directory.
 """
 from __future__ import absolute_import, division, print_function