David likely found this class when adding 3 fields in a row via the builder. Then, hovering over the row with Google developer tools.
That class specifically is automatically added when you add multiple fields to a row. It is not a class you add like a ready class.
Maybe GF just hasn’t had a chance to write an article about this yet.
If for informational purposes only you probably just have to wait a bit as I know there recent update has required a lot of adjustments.