x86, doc: Document that bootloader ID 4 is used also by iPXE

Etherboot -> gPXE -> iPXE...
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>
Cc: Michael Brown <mbrown@fensystems.co.uk>

Documentation/x86/boot.txt

@@ -373,7 +373,7 @@ Protocol:	2.00+
 	1  Loadlin
 	2  bootsect-loader	(0x20, all other values reserved)
 	3  Syslinux
-	4  Etherboot/gPXE
+	4  Etherboot/gPXE/iPXE
 	5  ELILO
 	7  GRUB
 	8  U-Boot