Clarify z2<z1 behavior
Clarify z2<z1 behavior

[ci skip]

Co-authored-by: Derek Homeier <955e65eba8e58aa01aa289170b3c5b72f3c4064d@gwdg.de>
larrybradley/astropy,pllim/astropy,dhomeier/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,pllim/astropy,mhvk/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,saimn/astropy,StuartLittlefair/astropy,astropy/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,astropy/astropy,saimn/astropy,pllim/astropy,pllim/astropy,larrybradley/astropy,dhomeier/astropy,astropy/astropy,mhvk/astropy,astropy/astropy,dhomeier/astropy,lpsinger/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,astropy/astropy,StuartLittlefair/astropy,mhvk/astropy,saimn/astropy,StuartLittlefair/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,pllim/astropy,saimn/astropy,dhomeier/astropy,mhvk/astropy,saimn/astropy,lpsinger/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy,dhomeier/astropy