Julia: Add doctest examples to some of the stdlib string functions

[kshyatt]

Scrolling through the documentation we can see that many of the string functions (especially ones involving regexes) don't have any doctest examples. Adding some would be a great way to get started if you're looking to contribute!

If the docstring mentions several possible behaviours of a function (for instance, if there's a kwarg) it's nice to provide examples for each of the described behaviours.

[fredrikekre]

Regex-related string docstrings could still use more examples.

[bkamins]

In #24039 I have added some additional doctests for strings.

[josobar]

I have added additional doctests in #34271

[KristofferC]

With all the work done, probably no need to keep this open.