docs: document CONTROL command of xenstore protocol

The CONTROL command (former DEBUG command) isn't specified in the
xenstore protocol doc. Add it.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Backport: 4.9+

docs/misc/xenstore.txt

@@ -318,12 +318,32 @@ SET_TARGET		<domid>|<tdomid>|
 
 ---------- Miscellaneous ----------
 
-DEBUG			print|<string>|??	    sends <string> to debug log
-DEBUG			print|<thing-with-no-nul>   EINVAL
-DEBUG			check|??		    checks xenstored innards
-DEBUG			<anything-else|>	    no-op (future extension)
+CONTROL			<command>|[<parameters>|]
+	Send a control command <command> with optional parameters
+	(<parameters>) to Xenstore daemon.
 
-	These requests should not generally be used and may be
-	withdrawn in the future.
+	The set of commands and their semantics is implementation
+	specific and is likely to change from one Xen version to the
+	next.  Out-of-tree users will encounter compatibility issues.
 
+	Current commands are:
+	check
+		checks xenstored innards
+	log|on
+		turn xenstore logging on
+	log|off
+		turn xenstore logging off
+	logfile|<file-name>
+		log to specified file
+	memreport|[<file-name>]
+		print memory statistics to logfile (no <file-name>
+		specified) or to specific file
+	print|<string>
+		print <string> to syslog (xenstore runs as daemon) or
+		to console (xenstore runs as stubdom)
+	help			<supported-commands>
+		return list of supported commands for CONTROL
+
+DEBUG
+	Deprecated, now named CONTROL