doc/advertising-api: Add Discoverable property

This adds Discoverable which the application can use in case it want
to set the General Discoverable flag per instance.

Note: This would allow for example an application to advertise as
discoverable even if the adapter is not discoverable which may be
required by dual-mode as it may not require BR/EDR to be discoverable.
diff --git a/doc/advertising-api.txt b/doc/advertising-api.txt
index eef98ad..96deeec 100644
--- a/doc/advertising-api.txt
+++ b/doc/advertising-api.txt
@@ -78,6 +78,14 @@
 				<Transport Discovery> <Organization Flags...>
 				0x26                   0x01         0x01...
 
+		bool Discoverable [Experimental]
+
+			Advertise as general discoverable. When present this
+			will override adapter Discoverable property.
+
+			Note: This property shall not be set when Type is set
+			to broadcast.
+
 		array{string} Includes
 
 			List of features to be included in the advertising