doc: patch-id: --verbatim locks in --stable

doc: patch-id: --verbatim locks in --stable

The default `--unstable` is a legacy format that predates `--stable`.
That’s why 2871f4d4 (builtin: patch-id: add --verbatim as a command mode,
2022-10-24) made `--verbatim` lock in[1] `--stable`:

    Users of --unstable mainly care about compatibility with old git
    versions, which unstripping the whitespace would break. Thus there
    isn't a usecase for the combination of --verbatim and --unstable,
    and we don't expose this so as to not add maintainence burden.

† 1: imply `--stable`, disallow `--unstable`

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