pvcalls: Clearly spell out that the header is just a reference

pvcalls: Clearly spell out that the header is just a reference

A recent thread on xen-devel [1] pointed out that the header was
provided as a reference for the specification.

Unfortunately, this was never written down in xen.git so for an external
user (or a reviewer) it is not clear whether the spec or the header
should be followed when there is a conflict.

To avoid more confusion, a paragraph is added at the top of the header
to clearly spell out it is only provided for reference.

[1] https://lore.kernel.org/xen-devel/alpine.DEB.2.21.2006151343430.9074@sstabellini-ThinkPad-T480s/

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Paul Durrant <paul@xen.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>