Docs: Reword and extend SeaBIOS description

The sentence about using SeaBIOS as secondary payload sounded
confusing, so reword it. While at it, improve and extend on SeaBIOS
features.

Change-Id: Ic06b9f56ab8082f2e6eff5fd8d31525429fd948d
Signed-off-by: Bill XIE <persmule@hardenedlinux.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64958
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
diff --git a/Documentation/payloads.md b/Documentation/payloads.md
index e34289c..9df6d64 100644
--- a/Documentation/payloads.md
+++ b/Documentation/payloads.md
@@ -11,11 +11,11 @@
 
 [SeaBIOS](https://www.seabios.org) is an open source implementation of
 the PCBIOS API that exists since the original IBM PC and was extended
-since. While originally written for emulators such as QEMU, it can be made
-to work as a coreboot payload and all the necessary code is in SeaBIOS'
-mainline code, or as a secondary payload load by another payload, e.g. it
-can be loaded from GRUB2 with the following menuentry in the run time
-config of GRUB2:
+since. While originally written for emulators such as QEMU, it can be built
+as a coreboot payload. It supports executing Option ROMs in a more complete
+fashion than coreboot. It also supports Multiboot.
+
+When chainloaded from GRUB2, the following menuentry could be used:
 
     menuentry "SeaBIOS" --unrestricted {
         root=(cbfsdisk)