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

The default `--unstable` is a legacy format that predates `--stable`.
That’s why 2871f4d (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>

Author: LemmingAvalanche

Documentation/git-patch-id.adoc

@@ -32,7 +32,7 @@ OPTIONS
 
 `--verbatim`::
 	Calculate the patch ID of the input as it is given, do not strip
-	any whitespace.
+	any whitespace. Implies `--stable` and forbids `--unstable`.
 +
 This is the default if `patchid.verbatim` is `true`.