[PATCH 1/3] docs/grub: Document serial port restrictions under lockdown

[PATCH 1/3] docs/grub: Document serial port restrictions under lockdown

Document that the serial command with the raw address is restricted
in lockdown mode to avoid misuse by an attacker (except for if the
given address is already associated with a platform provided
device).

Assisted-by: Claude:Opus-5
Signed-off-by: Andrew Hamilton <adhamilt@gmail.com>
Reviewed-by: Leo Sandoval <lsandova@redhat.com>
Part-of: <https://gitlab.freedesktop.org/gnu-grub/grub/-/merge_requests/266>

Gbp-Pq: Topic upstream
Gbp-Pq: Name 0001-docs-grub-Document-serial-port-restrictions-under-lo.patch