add comment explaining tolerance test [docs only]
add comment explaining tolerance test [docs only]

dhomeier/astropy,stargaser/astropy,larrybradley/astropy,stargaser/astropy,dhomeier/astropy,pllim/astropy,MSeifert04/astropy,dhomeier/astropy,mhvk/astropy,lpsinger/astropy,bsipocz/astropy,astropy/astropy,StuartLittlefair/astropy,stargaser/astropy,saimn/astropy,stargaser/astropy,saimn/astropy,astropy/astropy,saimn/astropy,pllim/astropy,saimn/astropy,StuartLittlefair/astropy,mhvk/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,aleksandr-bakanov/astropy,MSeifert04/astropy,bsipocz/astropy,lpsinger/astropy,bsipocz/astropy,larrybradley/astropy,pllim/astropy,dhomeier/astropy,mhvk/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,larrybradley/astropy,lpsinger/astropy,pllim/astropy,bsipocz/astropy,mhvk/astropy,saimn/astropy,MSeifert04/astropy,larrybradley/astropy,StuartLittlefair/astropy,larrybradley/astropy,astropy/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,mhvk/astropy,lpsinger/astropy,MSeifert04/astropy,astropy/astropy