updates to doc/modeling/compound.rst
updates to doc/modeling/compound.rst

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