Clarify that VecDeque::swap can panic

[daboross]

The previous documentation mentioned this, but ambiguously used the term "fail".

This clarifies that the function will panic if the index is out of bounds, instead of silently failing and not doing anything.

If there's anything else I can do to improve this PR, I'd be happy to do so! Just saw this when reading through the docs in passing - it was slightly unclear what "fail" meant.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

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[dtolnay]

Looks good to me. Thank you!

@bors r+

[bors]

📌 Commit f1b91f4 has been approved by dtolnay

[bors]

⌛ Testing commit f1b91f4 with merge 8460c81f910dc218cfa56549775ebbd5ad101860...

[bors]

💔 Test failed - status-travis

[kennytm]

@bors retry rollup

armhf-gnu failed while building LLVM.

[00:03:44] -- Constructing LLVMBuild project information
[00:03:44] -- Targeting X86
[00:03:44] -- Targeting ARM

No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.

Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

The build has been terminated

[bors]

⌛ Testing commit f1b91f4 with merge 5caca6f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: dtolnay
Pushing 5caca6f to master...