doc: add boot protocol 2.13 description to Documentation/x86/boot.txt

Documentation/x86/boot.txt is missing protocol 2.13 description.
Reported-by: Ross Philipson <ross.philipson@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: H. Peter Anvin <hpa@zytor.com>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/x86/boot.txt

@@ -61,6 +61,10 @@ Protocol 2.12:	(Kernel 3.8) Added the xloadflags field and extension fields
 		to struct boot_params for loading bzImage and ramdisk
 		above 4G in 64bit.
 
+Protocol 2.13:	(Kernel 3.14) Support 32- and 64-bit flags being set in
+		xloadflags to support booting a 64-bit kernel from 32-bit
+		EFI
+
 **** MEMORY LAYOUT
 
 The traditional memory map for the kernel loader, used for Image or