Added doc on prefix under Units
Added doc on prefix under Units

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