[docs] Group by clarifications

[hutch3232]

I was trying to reference these docs: https://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-munging/groupby.html

I think a few clarifications could be helpful. Some ideas:

1. Could an example of using the col.names gb.control arg be provided? I could not figure out how to do it. Saw this issue so perhaps it's not working? [R] group_by : specifying output col.names not working #12731
2. Under the "R only" header it shows nrow being an argument and the description refers to defining column names. That seems like a mistake.
3. Perhaps na.methods should be presented as a bullet underneath gb.control for clarity.
4. This note:

If a list smaller than the number of columns groups is supplied, then the list will be padded by ignore.

Could that be clarified? Is it saying the gb.control options are recycled? Could an example be provided where options are by-variable?

[wendycwong]

@hutch3232

An example code here:

So, sorry. Will fix. May take a while.