Update Gaussian2D theta docs
Update Gaussian2D theta docs

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