[clang-format] State where clang-format-diff.py should be run from

[DavidSpickett]

Summary

At least with git, file paths in a diff will be relative
to the repo root. So if you are in "llvm-project/lldb"
and the diff shows "clang/foo" modified you get:
No such file or directory

From clang-format-diff.py, since clang-format was
asked to read:
llvm-project/lldb/clang/foo

Add a note to the docs to explain this.

(there is git diff --relative but that excludes
changes outside of the current dir)

clang/docs/ClangFormat.rst

 
 .. code-block:: console
 
   svn diff --diff-cmd=diff -x -U0 | clang-format-diff.py -i
 
 The option `-U0` will create a diff without context lines (the script would format
 those as well).
 
+These commands use the file paths shown in the diff output
+so they will only work from the root of the repository.
+
 Current State of Clang Format for LLVM
 ======================================
 
 The following table :doc:`ClangFormattedStatus` shows the current status of clang-formatting for the entire LLVM source tree.