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

Commit 13dd372834 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: 13dd372834 ("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>
(cherry picked from commit 03bee3fd47)

docs/misc/xenstore.txt

@@ -294,6 +294,22 @@ IS_DOMAIN_INTRODUCED	<domid>|		T| or F|
 	ie, if INTRODUCE for the domain has not yet been followed by
 	domain destruction or explicit RELEASE.
 
+RESUME			<domid>|
+
+	Arranges that @releaseDomain events will once more be
+	generated when the domain becomes shut down.  This might have
+	to be used if a domain were to be shut down (generating one
+	@releaseDomain) and then subsequently restarted, since the
+	state-sensitive algorithm in xenstored will not otherwise send
+	further watch event notifications if the domain were to be
+	shut down again.
+
+	This command will be issued in place such as resume because
+	Xen will "shutdown" the domain on suspend.
+
+	xenstored prevents the use of RESUME other than by dom0.
+
+
 SET_TARGET		<domid>|<tdomid>|
 	Notifies xenstored that domain <domid> is targeting domain
 	<tdomid>. This grants domain <domid> full access to paths