write some documentation about the new functions
write some documentation about the new functions

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