☂ Provide KDocs for public API: Continued

Continuation of https://github.com/Kotlin/dataframe/issues/286.

Now we have a [KDoc preprocessing wiki/guide](https://github.com/Kotlin/dataframe/blob/master/KDOC_PREPROCESSING.md) (https://github.com/Kotlin/dataframe/issues/805), we can continue documenting the public API.

I'll make a small collection of operations that we can document together to start getting the hang of it. This will be an umbrella issue:

- https://github.com/Kotlin/dataframe/issues/814 (simple)
- https://github.com/Kotlin/dataframe/issues/817 (simple)
- https://github.com/Kotlin/dataframe/issues/819 (simple)
- https://github.com/Kotlin/dataframe/issues/1020 (not that simple)
- https://github.com/Kotlin/dataframe/issues/815 (a bit more advanced)
- https://github.com/Kotlin/dataframe/issues/813 (a bit more advanced)
- https://github.com/Kotlin/dataframe/issues/820 (a bit more advanced)
- https://github.com/Kotlin/dataframe/issues/1019 (a bit more advanced)
- https://github.com/Kotlin/dataframe/issues/816 (advanced)
- https://github.com/Kotlin/dataframe/issues/821 (very advanced)

If you want to use an operation as an example, I'd recommend looking at [select](https://github.com/Kotlin/dataframe/blob/master/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/select.kt).