presubmit: Update pydoc for AffectedFile.LocalPath()

To mention that it should not be used for accessing files.

Change-Id: I01ea79134bfc1f394c1791d47eb6fdb798358941
Reviewed-on: https://chromium-review.googlesource.com/750646Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>

presubmit_support.py

@@ -685,6 +685,10 @@ class AffectedFile(object):
 
   def LocalPath(self):
     """Returns the path of this file on the local disk relative to client root.
+
+    This should be used for error messages but not for accessing files,
+    because presubmit checks are run with CWD=PresubmitLocalPath() (which is
+    often != client root).
     """
     return normpath(self._path)