docs and tunable_boost_tree adjustments re: lightgbm bagging

[simonpcouch]

Some adjustments to effectively enable + document the sample_size argument for the lightgbm engine. Will be followed up by a PR in bonsai—1/2 to close tidymodels/bonsai#30.

The gist:

parsnip/man/rmd/boost_tree_lightgbm.Rmd

Lines 79 to 81 in 713d617

	The `sample_size` argument is translated to the `bagging_fraction` parameter in the `param` argument of `lgb.train`. The argument is interpreted by lightgbm as a _proportion_ rather than a count, so bonsai internally reparameterizes the `sample_size` argument with [dials::sample_prop()] during tuning.
	To effectively enable bagging, the user would also need to set the `bagging_freq` argument to lightgbm. `bagging_freq` defaults to 0, which means bagging is disabled, and a `bagging_freq` argument of `k` means that the booster will perform bagging at every `k`th boosting iteration. Thus, by default, the `sample_size` argument would be ignored without setting this argument manually. Other boosting libraries, like xgboost, do not have an analogous argument to `bagging_freq` and use `k = 1` when the analogue to `bagging_fraction` is in $(0, 1)$. _bonsai will thus automatically set_ `bagging_freq = 1` _in_ `set_engine("lightgbm", ...)` if `sample_size` (i.e. `bagging_fraction`) is not equal to 1 and no `bagging_freq` value is supplied. This default can be overridden by setting the `bagging_freq` argument to `set_engine()` manually.

Will PR as draft and we can come back to this after conf. :)

[simonpcouch]

Would be great to have this one in before the next release, @topepo!

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.