Add a note about `mag` unit.
Add a note about `mag` unit.

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