Allowed parameter documentation is ... sparse

[alexpghayes]

I had a really hard time figuring out what acceptable values are for parameters. Often times the documentation is unclear with regards to:

• Acceptable type
• Bounds on numerics

Beyond allowed values, some heuristics on what reasonable values are would also be valuable.

Occasionally categorical values aren't listed out either. Consider prune_method in mars, which only refers the user to earth::earth() without cross-linking the documentation.

On the whole, I strongly recommend a major update to the parameter documentation to add additional detail.

[topepo]

The current dev docs have more details