docs/about: Belatedly document tightening of QMP device_add checking

docs/about: Belatedly document tightening of QMP device_add checking

Commit 4d8b0f0a9536 (v6.2.0) deprecated incorrectly typed device_add
arguments.  Commit be93fd53723c (qdev-monitor: avoid QemuOpts in QMP
device_add) fixed them for v9.2.0, but neglected to update
documentation.  Do that now.

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250521063711.29840-2-armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[Commit message typo corrected]