diff: document behavior of relative diff.orderFile

Document that a relative pathname for diff.orderFile is interpreted as
relative to the top-level work directory.

Signed-off-by: Richard Hansen <hansenr@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -101,6 +101,8 @@ diff.noprefix::
 diff.orderFile::
 	File indicating how to order files within a diff, using
 	one shell glob pattern per line.
+	If `diff.orderFile` is a relative pathname, it is treated as
+	relative to the top of the working tree.
 	Can be overridden by the '-O' option to linkgit:git-diff[1].
 
 diff.renameLimit::