Doc tweak: explicitly state that vector contents are on the heap

[aidanhs]

...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?

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (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 The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

Please see CONTRIBUTING.md for more information.

[aidanhs]

Re-wrapped to 99 chars per style.

[brson]

@aidanhs The issue of updating the copyright date is pretty fuzzy. It's kind of a best-effort thing. Personally I usually don't bother. I'm not sure whether we should change the contributing file or not.

[aidanhs]

I guess if it needs to be done eventually then it can be left in there.