git-diff.txt: group the [--] and [<path>...] templates

Clarify that "--" is meant to disambiguate paths from the other
options.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-diff.txt: document the .. and ... forms in SYNOPSIS

The DESCRIPTION refers to the '<commit>..<commit>' and
'<commit>...<commit>' forms, but the SYNOPSIS does not list them at
all.  Fix this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-diff.txt: fix ordering of the [--no-index] form

The SYNOPSIS lists the [--no-index] form as the last item, but the
DESCRIPTION lists it as a natural extension of the first form.  Fix
this with a reordering.  Also since the DESCRIPTION breaks up the
first form in the SYNOPSIS into two different forms (one without the
optional [<commit>], and the other with it), reorder to make these two
forms appear together.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>