doc/advertising-api: Add DiscoverableTimeout property

This property can control how long an instance is visible.
diff --git a/doc/advertising-api.txt b/doc/advertising-api.txt
index 96deeec..cfce0f9 100644
--- a/doc/advertising-api.txt
+++ b/doc/advertising-api.txt
@@ -86,6 +86,15 @@
 			Note: This property shall not be set when Type is set
 			to broadcast.
 
+		uint16 DiscoverableTimeout [Experimental]
+
+			The discoverable timeout in seconds. A value of zero
+			means that the timeout is disabled and it will stay in
+			discoverable/limited mode forever.
+
+			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