Documentation and implementation of directives flag don't match

[davidben]

The documentation says that prove --directives and the corresponding argument to TAP::Harness override a choice of verbose and failures.

If set to a true value, only test results with directives will be displayed. This overrides other settings such as verbose or failures.

However, the current implementation allows passing --directives --failures and it will print both (as is reasonable). It appears the implementation once matched the description, but this got flipped in a2542a9. The documentation probably should be updated to match. (Or implementation restored to original logic, but I think the current logic makes much more sense. Otherwise you can't see both skipped and failed tests at the same time.)