Update group_by / aggregate docstrings
Update group_by / aggregate docstrings

stargaser/astropy,tbabej/astropy,astropy/astropy,kelle/astropy,DougBurke/astropy,bsipocz/astropy,dhomeier/astropy,kelle/astropy,AustereCuriosity/astropy,saimn/astropy,astropy/astropy,StuartLittlefair/astropy,stargaser/astropy,MSeifert04/astropy,kelle/astropy,saimn/astropy,pllim/astropy,joergdietrich/astropy,lpsinger/astropy,saimn/astropy,stargaser/astropy,astropy/astropy,lpsinger/astropy,bsipocz/astropy,DougBurke/astropy,pllim/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,StuartLittlefair/astropy,mhvk/astropy,joergdietrich/astropy,astropy/astropy,AustereCuriosity/astropy,saimn/astropy,larrybradley/astropy,MSeifert04/astropy,lpsinger/astropy,tbabej/astropy,saimn/astropy,StuartLittlefair/astropy,astropy/astropy,joergdietrich/astropy,kelle/astropy,funbaker/astropy,larrybradley/astropy,mhvk/astropy,MSeifert04/astropy,bsipocz/astropy,mhvk/astropy,aleksandr-bakanov/astropy,pllim/astropy,tbabej/astropy,funbaker/astropy,funbaker/astropy,kelle/astropy,dhomeier/astropy,lpsinger/astropy,stargaser/astropy,dhomeier/astropy,AustereCuriosity/astropy,StuartLittlefair/astropy,pllim/astropy,DougBurke/astropy,tbabej/astropy,mhvk/astropy,tbabej/astropy,aleksandr-bakanov/astropy,AustereCuriosity/astropy,larrybradley/astropy,AustereCuriosity/astropy,MSeifert04/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy,dhomeier/astropy,joergdietrich/astropy,bsipocz/astropy,joergdietrich/astropy,dhomeier/astropy,mhvk/astropy,DougBurke/astropy,aleksandr-bakanov/astropy,funbaker/astropy,StuartLittlefair/astropy