added check and doc clarification in configs._generate_all_config_items
added check and doc clarification in configs._generate_all_config_items

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