doc: diff-options.adoc: show format.noprefix for format-patch

doc: diff-options.adoc: show format.noprefix for format-patch

git-format-patch(1) uses `format.noprefix` and ignores `diff.noprefix`.

The configuration variable `format.prefix` was added as an “escape
hatch”, and “it’s unlikely that anybody really wants format.
noprefix=true in the first place.”[1] Based on that there doesn’t
seem to be a need to widely advertise this configuration variable.

But in any case: the documentation for this option should not claim
that it overrides a config that is always ignored.

† 1: 8d5213de (format-patch: add format.noprefix option, 2023-03-09)

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>