Improve combine documentation

[bkamins]

source https://discourse.julialang.org/t/normalizing-dataframe-column-by-group/89226/5

the help accessible by typing ?combine states “combine: does not put restrictions on number of rows returned, the order of rows is specified by the order of groups in GroupedDataFrame…” which doesn’t specify exactly how the groups are combined. I think adding a description something like “combine() applies a function to each group in a GroupedDataFrame and concatenates the results into a single DataFrame” would be helpful.