add Vallado ref to docstring
add Vallado ref to docstring

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