Documentation: Add method for GRUB2 to load seabios from drive

Confirmed on an image with GRUB2 payload and without seabios as
secondary payload.

Signed-off-by: Ivan Kuzneczov <ivan.kuzneczov@hardenedvault.net>
Change-Id: I093a9b9e8cabe6b21ec9d755a1592438209a86c6
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90658
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

Documentation/payloads.md

@@ -23,6 +23,12 @@ When chainloaded from GRUB2, the following menuentry could be used:
         module /vgaroms/seavgabios.bin
     }
 
+GRUB2 loading seabios from drive via command prompt is also possible:
+
+    multiboot (usb0,msdos1)/seabios/bios.bin.elf
+    module (usb0,msdos1)/seabios/vgabios.bin name=vgaroms/seavgabios.bin
+    boot
+
 ## edk2
 
 [edk2](https://github.com/tianocore/tianocore.github.io/wiki/Getting-Started-with-EDK-II) is an open-source modern, feature-rich,