Clarify xgboost mtry default

[jxu]

I've looked in dials and parsnip and I can't find what mtry actually defaults to. Does it with count=F correspond to xgboost's colsample_bynode?
I believe for random forests, mtry default is sqrt(# features), but xgboost by default uses all columns.

[hfrick]

Thanks for the issue @jxu ! In boost_tree(), the default value for mtry is NULL. This is passed as 1 for colsample_bynode to xgboost::xgb.train(). We'll note that in the documentation 👍

[github-actions]

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