Make it clear that push is only amortized O(1)

[brianquinlan]

No description provided.

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[huonw]

@bors r+

Thanks!

[bors]

📌 Commit 7389b0a has been approved by huonw

[Gankra]

Our guarantee is, strictly speaking, a bit stronger than that: if you can accurately reserve a sufficient length, then it is O(1) worst-case. Ah well. 6 of one, half-a-dozen of the other.