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>