Add more tests and comments for Godoc

plugin/const.go

@@ -4,8 +4,11 @@
 package plugin
 
 const (
+	// ConstListInstancesFilter refers to a Google Cloud SDK filter to search for instances
 	ConstListInstancesFilter = "filter"
-	ConstInstanceGroup       = "instance_group"
+
+	// ConstInstanceGroup refers to the name of a Google Cloud instance group
+	ConstInstanceGroup = "instance_group"
 )
 
 var allowedSetFields = map[string]struct{}{