Add general docstring.
Add general docstring.

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