More sparse models documentation (#1254)

* mention that ranger doesn't care about sparse data

* mention that lightgbm can use sparse data

Author: EmilHvitfeldt

man/rmd/boost_tree_lightgbm.Rmd

@@ -84,6 +84,11 @@ To effectively enable bagging, the user would also need to set the `bagging_freq
 
 bonsai quiets much of the logging output from [lightgbm::lgb.train()] by default. With default settings, logged warnings and errors will still be passed on to the user. To print out all logs during training, set `quiet = TRUE`.
 
+## Sparse Data
+
+```{r child = "template-uses-sparse-data.Rmd"}
+```
+
 ## Examples 
 
 The "Introduction to bonsai" article contains [examples](https://bonsai.tidymodels.org/articles/bonsai.html) of `boost_tree()` with the `"lightgbm"` engine.

man/rmd/rand_forest_ranger.Rmd

@@ -77,6 +77,8 @@ For `ranger` confidence intervals, the intervals are  constructed using the form
 ```{r child = "template-uses-sparse-data.Rmd"}
 ```
 
+While this engine supports sparse data as an input, it doesn't use it any differently than dense data. Hence there it no reason to convert back and forth.
+
 ## Saving fitted model objects
 
 ```{r child = "template-butcher.Rmd"}