Document in strings.rst usage of special_dtype(vlen=str)

[doronbehar]

I have been using the example I documented here for a while now. Felt it was missing. I had to learn this from the question here.

[aragilar]

Rather than h5py.special_dtype(vlen=str) (which is oldish and somewhat implicit about what it means, though I don't think we'll drop support for it), you should use h5py.string_dtype(encoding='utf-8', length=None) (or, since those are the default values h5py.string_dtype()).

[codecov]

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  Coverage   89.58%   89.58%           
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[doronbehar]

Hmm I wonder how I missed that back then... The docs indeed mention [string_dtype](https://docs.h5py.org/en/stable/special.html#h5py.string_dtype), but they don't tell you to use that for datasets, and only gives examples for attributes. I pushed something.

[takluyver]

Thanks! I think this could benefit from an example. But it's a fairly confusing topic, because of all the different possible string types, and I think it needs careful presentation to avoid muddying the waters. So I hope you won't mind if I open a new PR reworking this example to show the different possibilities.

[takluyver]

I've tried to do that in #2424.