add description of z_at_value along with some other minor clean-ups
add description of z_at_value along with some other minor clean-ups

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