Add KDocs for `gather`

Related to https://github.com/Kotlin/dataframe/issues/811

The [gather operation](https://kotlin.github.io/dataframe/gather.html) is also similar to update in terms of how to add KDocs to it; users select a bunch of columns (we have templates for this concept), there's a lot of overloads with the same name, and there's a nice DSL grammar opportunity.

