Clarify use of cds units [skip ci]
Clarify use of cds units [skip ci]

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