shill: Add missing documentation for the default service manager property

BUG=none
TEST=none

Change-Id: I411b4b46fb43a6084dfc64d5bb367f57c991c8f4
Reviewed-on: https://gerrit.chromium.org/gerrit/37405
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Tested-by: Gaurav Shah <gauravsh@chromium.org>
diff --git a/doc/manager-api.txt b/doc/manager-api.txt
index 15ede7a..a6da636 100644
--- a/doc/manager-api.txt
+++ b/doc/manager-api.txt
@@ -315,6 +315,10 @@
 			it is defined if manually set or if it is discovered
 			through a service such as WiFi, Celluar, or GPS.
 
+		string DefaultService [readonly]
+
+			The current connected service with the default route.
+
 		string DefaultTechnology [readonly]
 
 			The current connected technology which holds the