DOC: Add known issue with np.arange() and units
DOC: Add known issue with np.arange() and units

See #11582.

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