docs: clarify Xenstore watch depth feature

Make it explicit that for watching the @releaseDomain/<domid> special
node, the XENSTORE_SERVER_FEATURE_WATCHDEPTH is needed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>

docs/misc/xenstore.txt

@@ -266,7 +266,8 @@ WATCH			<wpath>|<token>|[<depth>|]?
 	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.).
+	etc.).  The depth specification is possible only, if the
+	XENSTORE_SERVER_FEATURE_WATCHDEPTH feature is available.
 
 	<wpath> can be a <path> to watch or @<wspecial>.  In the
 	latter case <wspecial> may have any syntax but it matches
@@ -284,6 +285,8 @@ WATCH			<wpath>|<token>|[<depth>|]?
 	values are not supported.
 	For @releaseDomain it is possible to watch only for a specific
 	domain by specifying @releaseDomain/<domid> for the path.
+	This requires the XENSTORE_SERVER_FEATURE_WATCHDEPTH to be
+	available.
 
 	When a watch is first set up it is triggered once straight
 	away, with <path> equal to <wpath>.  Watches may be triggered