docs/misc: xenstored: Re-instate and tweak the documentation for XS_RESUME

docs/misc: xenstored: Re-instate and tweak the documentation for XS_RESUME

Commit 13dd372834a4 removed the documentation for XS_RESUME, however
this command is still implemented (at least in C Xenstored) and used by
libxl when resuming a domain.

So re-instate the documentation for the XS_RESUME. Take the opportunity
to update it as there is a user of the command.

Fixes: 13dd372834a4 ("docs/designs: re-work the xenstore migration document...")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>