Document which functions work with grouped data

[gregorp]

On the ?fill help page, there is no statement that fill knows to respect groups from dplyr::group_by. (The last example does demonstrate this.) Same on the ?expand and ?complete help pages, possibly other functions too. It would be great if there was a standard sentence or phrase that was part of the Description section for all non-dplyr functions that know what to do with a grouped_df. Perhaps "Respects tibble groupings".

I think this would both serve as a good reference (I constructed a little example before posting thing to check on expand... I was pretty sure it respected groups but needed to check to see), and also for those new users who read help pages it might help cement the concept that group_by isn't magic that works with every function, only specially constructed functions.

[DavisVaughan]

#1300 mentions in the docs that complete() and expand() work with grouped data frames, and adds examples