diff --no-index: describe in a separate paragraph

The documentation for "diff-files" mode of "git diff" primarily
talks about how changes in the files in the working tree are shown
relative to the contents previously added to that index, and tucks
explanation on how "--no-index" mode, which works in a quite
different way, may be implicitly used instead.  Instead, add a
separate paragraph to explain what "--no-index" mode does, and also
mention when "--no-index" can be omitted from the command line
(essentially, when it is obvious from the context).

Signed-off-by: Junio C Hamano <gitster@pobox.com>