Documentation/admin-guide: Document nomodeset kernel parameter

The nomodeset kernel command line parameter is not documented. Its name
is quite vague and is not intuitive what's the behaviour when it is set.

Document in kernel-parameters.txt what actually happens when nomodeset
is used. That way, users could know if they want to enable this option.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211112133230.1595307-6-javierm@redhat.com