GODRIVER-888 update GoDoc example for cursor

Change-Id: Ie95f667da2814e292f68c874eef89387327174f7

Author: iwysiu

mongo/doc.go

@@ -35,12 +35,21 @@
 //    if err != nil { log.Fatal(err) }
 //    defer cur.Close(context.Background())
 //    for cur.Next(context.Background()) {
-//       raw, err := cur.DecodeBytes()
-//       if err != nil { log.Fatal(err) }
-//       // do something with elem....
+//      // To decode into a struct, use cursor.Decode()
+//      result := struct{
+//        Foo string
+//        Bar int32
+//      }{}
+//      err := cur.Decode(&result)
+//      if err != nil { log.Fatal(err) }
+//      // do something with result...
+//
+//      // To get the raw bson bytes use cursor.Current
+//      raw := cur.Current
+//      // do something with raw...
 //    }
 //    if err := cur.Err(); err != nil {
-//    		return err
+//      return err
 //    }
 //
 // Methods that only return a single document will return a *SingleResult, which works