Documented caveat of the remove_dir_all

[tailhook]

I'm not sure the wording is great.

Maybe it's just a bug that is going to be fixed because the current behavior is probably useless. But if it is, it's a backwards-incompatible change.

[rust-highfive]

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[alexcrichton]

I think we'd be fine actually fixing bugs like this, I would prefer to not document "obviously wrong" behavior and instead just fix the bug in question.

[tailhook]

Okay. Just submitted a butfix pull request.