Update VOTable invalid unit string warning
Update VOTable invalid unit string warning

The unit format of a VOTable is version-dependent. The docstring of the
VOTable invalid unit string warning (W50) should therefore contain
references to both the old and the new unit format specification.

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