docs/design: Update xenstore-migration.md

docs/design: Update xenstore-migration.md

It is not very clear the shared page adddress is not contained in the
connection record. Additionally, it is misleading to say the grant
will always point to the share paged as a domain is free to revoke the
permission. The restore code would need to make sure it doesn't
fail/crash if this is happening.

The sentence is now replaced with a paragraph explaining why the GFN is
not preserved and that the grant is not guarantee to exist during
restore.

Take the opportunity to replace "code" with "node" when description the
permission.

Reported-by: Raphael Ning <raphning@amazon.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>