Documentation/diff-index: mention two modes of operation

Documentation/diff-index: mention two modes of operation

"diff-index" can be used to compare a tree with the tracked working
tree files (when used without the --index option), or with the index
(when used with the --index option).

The text however did not say anything about the comparison with the
working tree at all.  Fix this.

Reported-by: Albert Netymk <albertnetymk@gmail.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>