Document optional usage of data argument in `set_fit()`

[EmilHvitfeldt]

set_fit() has 4 mandatory arguments interface, protect, func, and defaults. It also have a optional argument as seen in usage here:

https://github.com/tidymodels/parsnip/blob/ed346749c645b6533b1c94863aa4a1f0fcf7713e/R/bart_data.R#L45-L56

This is not documented here: https://www.tidymodels.org/learn/develop/models/