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

Documentation/admin-guide/kernel-parameters.txt

@@ -3545,6 +3545,13 @@
 			shutdown the other cpus.  Instead use the REBOOT_VECTOR
 			irq.
 
+	nomodeset	Disable kernel modesetting. DRM drivers will not perform
+			display-mode changes or accelerated rendering. Only the
+			system framebuffer will be available for use if this was
+			set-up by the firmware or boot loader.
+
+			Useful as fallback, or for testing and debugging.
+
 	nomodule	Disable module load
 
 	nopat		[X86] Disable PAT (page attribute table extension of