doc: update the supplementary services docs

Document the various SS control string related APIs

doc/supplementaryservices-api.txt

@@ -7,15 +7,22 @@ Object path	[variable prefix]/{modem0,modem1,...}
 
 Methods		string, variant Initiate(string command)
 
-			Sends a USSD command string to the network
-			initiating a session.  When the request is handled
+			If the command is a recognized supplementary service
+			control string, the corresponding SS request is made
+			and the result is returned.
+
+			Otherwise the command is sent to the network
+			initiating a USSD session. When the request is handled
 			by the appropriate node of the network, the
 			method returns the response or an appropriate
-			error.  The network may be awaiting further response
+			error. The network may be awaiting further response
 			from the ME after returning from this method and no
 			new command can be initiated until this one is
 			cancelled or ended.
 
+			The output arguments are described in section
+			"Initiate method outptut arguments" below.
+
 			Possible Errors: [service].Error.Timedout
 			Possible Errors: [service].Error.Canceled
 
@@ -68,3 +75,93 @@ Properties	string State [readonly]
 			"user-response"		The network is waiting for the
 						user's response, client must
 						call Respond().
+
+
+Initiate method output arguments
+================================
+
+The first return argument and the corresponding second return argument are:
+
+	"USSD"				string ussd_response
+	"CallBarring"			(string ss_op, string cb_service,
+					dict cb_dict)
+	"CallForwarding"		(string ss_op, string cf_service,
+					dict cf_dict)
+	"CallWaiting"			(string ss_op, dict cw_dict)
+	"CallingLinePresentation"	(string ss_op, string status)
+	"CalledLinePresentation"	(string ss_op, string status)
+	"CallingLineRestriction"	(string ss_op, string clir_status)
+	"CalledLineRestriction"		(string ss_op, string status)
+
+ss_op contains the supplementary service operation:
+
+	"activation"
+	"registration"
+	"interrogation"
+	"deactivation"
+	"erasure"
+
+cb_service contains the call barring service for which the operation was
+requested:
+
+	"AllOutgoing"
+	"InternationalOutgoing"
+	"InternationalOutgoingExceptHome"
+	"AllIncoming"
+	"IncomingWhenRoaming"
+	"AllBarringServices"
+	"AllOutgoingServices"
+	"AllIncomingServices"
+
+cf_service contains the call forwarding service for which the operation was
+requested:
+
+	"Unconditional"
+	"Busy"
+	"NoReply"
+	"NotReachable"
+	"All"
+	"AllConditional"
+
+cb_dict contains basic service/call barring service combinations that were
+affected by SS operation and their current status ("enabled" or "disabled").
+The basic services are:
+
+	"Voice"
+	"Data"
+	"Fax"
+	"Sms"
+	"DataSync"
+	"DataAsync"
+	"DataPad"
+	"DataPacket"
+
+To those the name of call barring service is appended, so the property and
+value is for example:
+
+	"FaxIncomingWhenRoaming" : "disabled"
+
+cf_dict contains call forwarding properties affected by the operation.
+Propery names are formed from basic service name and call forwarding
+service name, for example:
+
+	"VoiceNoReply" : "+12345678"
+
+The property value is the phone number to which the call is forwarded.
+
+For "NoReply" service, there is also a timeout property, holding the timeout
+in seconds, for example:
+
+	"VoiceNoReplyTimeout" : 20
+
+cw_dict contains basic services with "CallWaiting" suffix that were affected
+by call waiting operation and their current status ("enabled" or "disabled"),
+for example:
+
+	"VoiceCallWaiting" : "enabled"
+
+status can be "disabled" or "enabled".
+clir_status can be "disabled", "permanent", "on" or "off".
+
+More information about supplementary services is provided in
+call-barring-api.txt, call-forwarding-api.txt and call-settings-api.txt