doc tweak to clarify warning about config file default building process
doc tweak to clarify warning about config file default building process

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