tools/xenstore: add documentation for extended watch command

Add documentation for an extension of the WATCH command used to limit
the scope of watched paths. Additionally it enables to receive more
information in the events related to special watches (@introduceDomain
or @releaseDomain).

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

docs/misc/xenstore-ring.txt

@@ -69,6 +69,7 @@ Bit     Description
 -----------------------------------------------------------------
 0       Ring reconnection (see the ring reconnection feature below)
 1       Connection error indicator (see connection error feature below)
+2       WATCH can take a third parameter limiting its scope
 
 The "Connection state" field is used to request a ring close and reconnect.
 The "Connection state" field only contains valid data if the server has

docs/misc/xenstore.txt

@@ -176,7 +176,7 @@ SET_PERMS		<path>|<perm-as-string>|+?
 
 ---------- Watches ----------
 
-WATCH			<wpath>|<token>|?
+WATCH			<wpath>|<token>|[<depth>|]?
 	Adds a watch.
 
 	When a <path> is modified (including path creation, removal,
@@ -187,7 +187,11 @@ WATCH			<wpath>|<token>|?
 	matching watch results in a WATCH_EVENT message (see below).
 
 	The event's path matches the watch's <wpath> if it is an child
-	of <wpath>.
+	of <wpath>. This match can be limited by specifying <depth> (a
+	decimal value of 0 or larger): it denotes the directory levels
+	below <wpath> to consider for a match ("0" would not match for
+	a child of <wpath>, "1" would match only for a direct child,
+	etc.).
 
 	<wpath> can be a <path> to watch or @<wspecial>.  In the
 	latter case <wspecial> may have any syntax but it matches
@@ -198,7 +202,13 @@ WATCH			<wpath>|<token>|?
 				shutdown, and also on RELEASE
 				and domain destruction
 	<wspecial> events are sent to privileged callers or explicitly
-	via SET_PERMS enabled domains only.
+	via SET_PERMS enabled domains only. The semantics for a
+	specification of <depth> differ for generating <wspecial>
+	events: specifying "1" will report the related domid by using
+	@<wspecial>/<domid> for the reported path. Other <depth>
+	values are not supported.
+	For @releaseDomain it is possible to watch only for a specific
+	domain by specifying @releaseDomain/<domid> for the path.
 
 	When a watch is first set up it is triggered once straight
 	away, with <path> equal to <wpath>.  Watches may be triggered