Skip argument of doctest-skip in the sphinx output

The doctest-skip argument was introduced in https://github.com/astropy/astropy/pull/2459 for the test itself, however it isn't recognized by sphinx when generating the docs. 

see the 'win32' text left behind in these examples:
http://docs.astropy.org/en/latest/table/operations.html#identical-keys

I can't test on windows, but I think this shouldn't brake anything there.