DOC: Flesh out the R comparison section of docs (GH3980)

Some additions to #3980. 

I've done some more comparisons to the plyr functions located [here](http://nbviewer.ipython.org/gist/chappers/8066230/). More specifically [aaply](http://nbviewer.ipython.org/gist/chappers/8066230/aaply.ipynb), [alply](http://nbviewer.ipython.org/gist/chappers/8066230/alply.ipynb) don't really have much to do with pandas (its just list comprehension). and [dlply](http://nbviewer.ipython.org/gist/chappers/8066230/dlplyr.ipynb) is probably more suited to be in statsmodel example rather than pandas.