doc/device-api: Add AdvertisingFlags

This exposed Advertising Flags to BlueZ Device API
diff --git a/doc/device-api.txt b/doc/device-api.txt
index f5cac49..13b2881 100644
--- a/doc/device-api.txt
+++ b/doc/device-api.txt
@@ -230,3 +230,7 @@
 
 			Indicate whether or not service discovery has been
 			resolved.
+
+		array{byte} AdvertisingFlags [readonly, experimental]
+
+			The Advertising Data Flags of the remote device.