Document workaround on `main_binary` first run (#57)

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#57 / #4 is about a performance vs correctness issue in assert_cmd. We should document the workaround for people who want the performance at the cost of correctness (or rather, they can ensure the correctness).

In how we document this: I noticed #6's documented workaround is in the cargo module. The challenge is if people look at the trait or functions directly. We should find a way to address that for that workaround and this one.

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We should also document the lack of features control. See #59.

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Now documented as of #62