Sigma clip doc improvements
Sigma clip doc improvements

astropy/astropy,larrybradley/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,lpsinger/astropy,lpsinger/astropy,saimn/astropy,mhvk/astropy,astropy/astropy,larrybradley/astropy,pllim/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,StuartLittlefair/astropy,saimn/astropy,lpsinger/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,larrybradley/astropy,StuartLittlefair/astropy,mhvk/astropy,larrybradley/astropy,saimn/astropy,mhvk/astropy,astropy/astropy,mhvk/astropy,saimn/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,pllim/astropy