Auto merge of #2281 - rillian:doc-nocapture, r=alexcrichton
Auto merge of #2281 - rillian:doc-nocapture, r=alexcrichton

Although this is a feature of rustc's test harness and
not cargo's test driver, I can never remember how to
invoke it. Describing the switch in `cargo test --help`
makes it more discoverable.

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