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>