doc: document BasicServiceSet API

doc/basic-service-set.txt

@@ -0,0 +1,10 @@
+Basic service set hierarchy
+=================
+
+Service		net.connman.iwd
+Interface	net.connman.iwd.BasicServiceSet
+Object path	/net/connman/iwd/{phy0,phy1,...}/{1,2,...}/Xxx
+
+Properties	string Address [readonly]
+
+			MAC address of BSS

doc/network-api.txt

@@ -50,3 +50,8 @@ Properties	string Name [readonly]
 			corresponding to this Network.  If the network
 			is not provisioned or has not been connected to
 			before, the property is omitted.
+
+		array(object) ExtendedServiceSet [readonly]
+
+			Contains a list of paths of each individual
+			BasicServiceSet object.

doc/station-api.txt

@@ -164,6 +164,12 @@ Properties	string State [readonly]
 			for networks.  net.connman.iwd.Network objects are
 			updated when this property goes from true to false.
 
+		object ConnectedAccessPoint [readonly, optional]
+
+			net.connman.iwd.BasicServiceSet object represeting the
+			BSS the device is currently connected to or to which
+			a connection is in progress.
+
 SignalLevelAgent hierarchy
 ==========================