Fix documentation build warnings
Fix documentation build warnings

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