DOC: add examples to groupby.first
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Add a few more examples to `groupby.first`. I also removed the mention of `numeric_only = None` since the code doesn't check for None any case, nor does the type allow for it.
Hello @Moisan! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-04-24 14:36:17 UTC doesn't this fix the same as #46792 ? though this fixes for groupby i think that is fixed as well. https://github.com/pandas-dev/pandas/pull/46792 covers unused arguments in `Resampler.first` however `GroupBy.first` doc doesn't include unused arguments so I don't think that PR is related. @Moisan could you merge in the main branch one more time? After that LGTM thanks @Moisan 