Update stat.funcs API docs to indicate optional keywords [skip ci]
Update stat.funcs API docs to indicate optional keywords [skip ci]

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