change spectral density section to include a luminosity density example
change spectral density section to include a luminosity density example

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