Documentation: update implicit "--no-index" behavior in "git diff"

Originally "--no-index" mode triggered for untracked files within the
tracked tree, but with v1.5.6-rc1~41 (Merge branch 'jc/diff-no-no-index,
2008-05-26) the command was fixed to only implicitly trigger when paths
outside the tracked tree are mentioned.
Reported-by: NYann Dirson <dirson@bertin.fr>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -23,9 +23,9 @@ tree and the index file, or the index file and the working tree.
 	further add to the index but you still haven't.  You can
 	stage these changes by using linkgit:git-add[1].
 +
-If exactly two paths are given, and at least one is untracked,
-compare the two files / directories. This behavior can be
-forced by --no-index.
+If exactly two paths are given and at least one points outside
+the current repository, 'git diff' will compare the two files /
+directories. This behavior can be forced by --no-index.
 
 'git diff' [--options] --cached [<commit>] [--] [<path>...]::