Add documentation and unittests regarding multisets - take #3

[RazvanN7]

Applied review from #5643.

CC @andralex

[dlang-bot]

Thanks for your pull request, @RazvanN7! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.

Some tips to help speed things up:

• smaller, focused PRs are easier to review than big ones

• try not to mix up refactoring or style changes with bug fixes or feature enhancements

• provide helpful commit messages explaining the rationale behind each change

Bear in mind that large or tricky changes may require multiple rounds of review and revision.

Please see CONTRIBUTING.md for more information.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[MetaLang]

LGTM

[andralex]

thx!

[wilzbach]

Yeah, all good things are three!

[andralex]

I swear I didn't search the Net for what follows: "Alle gute Dinge sind drei"...?

[andralex]

ok off by a bit...