Use False, instead of None as default for _XfailMarkDecorator's condition param and update doc

[Avasam]

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Fixes comments pypa/setuptools#3979 (comment) and #10094 (comment)

This makes it explicit that None is not a contractually supported value (even if it "works" because it's falsy). See #11565 (comment)

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[nicoddemus]

Thanks @Avasam, LGTM! 👍

@RonnyPfannschmidt can you take a look too please?