rollup merge of #21168: aidanhs/aphs-doc-vec-heap

...to make it slightly clearer that there's not much point in boxing a vec.

On a different note, I read the contribution guidelines (https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#pull-request-procedure) which say I should update the copyright date for this file. But I can see that nobody else has done this so far this year, despite there being a fair number of commits.

Does that instruction need removing?

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