[std.container] Improve docs

[ntrel]

See commit messages.

[dlang-bot]

Thanks for your pull request and interest in making D better, @ntrel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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Testing this PR locally

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dub run digger -- build "master + phobos#10860"

[ntrel]

Not sure why auto-merge didn't work, tests passed. So I merged it myself only because auto-merge had been set.