public/io: xs_wire: Document that new errors should be added at the end

public/io: xs_wire: Document that new errors should be added at the end

Some tools (e.g. xenstored) always expect EINVAL to be first in xsd_errors.

To be conservative, mandate that new errors should be added at the end
of the array.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>