Fix Marshal documentation on omitempty and IsZero (#608)

yaml.go

@@ -175,7 +175,7 @@ func unmarshal(in []byte, out interface{}, strict bool) (err error) {
 //                  Zero valued structs will be omitted if all their public
 //                  fields are zero, unless they implement an IsZero
 //                  method (see the IsZeroer interface type), in which
-//                  case the field will be included if that method returns true.
+//                  case the field will be excluded if IsZero returns true.
 //
 //     flow         Marshal using a flow style (useful for structs,
 //                  sequences and maps).