doc: am: revert Message-ID trailer claim

doc: am: revert Message-ID trailer claim

I claimed in 3c18135b (doc: am: say that --message-id adds a trailer,
2026-02-09) that `git am --message-id` adds a Git trailer. But that
isn’t the case; for the case of a commit message with a subject, body,
and no trailer block:

    <subject>

    <paragrah>

It just appends the line right after `paragraph`:

    <subject>

    <paragraph>
    Message-ID: <message-id_trailer.323@msgid.xyz>

It does work for two other cases though, namely subject-only and with an
existing trailer block.

This is at best an inconsistency and arguably a bug, but we’re at the
trailing end of the release cycle now. So reverting the doc is safer
than making msg-id act as a trailer, for now.

Revert this hunk from commit 3c18135b except the only useful
change (“Also use inline-verbatim for `Message-ID`”).

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