add documentation for switching the default parameter set
add documentation for switching the default parameter set

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