Updated the new model docs to better reflect the updated custom_model implementation.
Updated the new model docs to better reflect the updated custom_model implementation.

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