☂ Provide KDocs for public API: Continued

[Jolanrensen]

Continuation of #286.

Now we have a KDoc preprocessing wiki/guide (#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:

• Add KDocs for flatten #814 (simple)
• Add KDocs for distinct/distinctBy #817 (simple)
• Add KDocs for remove #819 (simple)
• Add KDocs for corr #1020 (not that simple)
• Add KDocs for gather #815 (a bit more advanced)
• Add KDocs for move #813 (a bit more advanced)
• Add KDocs for group/ungroup #820 (a bit more advanced)
• Add KDocs for convert #1019 (a bit more advanced)
• Add KDocs for add #816 (advanced)
• Add KDocs for groupBy/pivot #821 (very advanced)

If you want to use an operation as an example, I'd recommend looking at select.