Add KDocs for `add`

Related to https://github.com/Kotlin/dataframe/issues/811

The [add operation](https://kotlin.github.io/dataframe/add.html) already has some KDocs here and there, however, it does not yet use the preprocessor, it has no DSL grammar yet (which it could really use), and it's one of the most used functions in the library (AFAIK). It's a bit more difficult task though.