[docs] [issue13828] Further improve casefold documentation

[Marc Richter]

Marc Richter <marc.richter.1982 at googlemail.com> added the comment:

+1 as well.

To be honest, I did not understand what this function does in detail yet.
Since not too long ago (2017) in Germany, there was an uppercase-variant for the special letter from this function's example (ß) been added to the official orthography [1].
Is this something that needs to be changed in this function's behavior now or stays this expected behavior? I'm still puzzled and I think the whole function should get a more clear description.

[1]: https://en.wikipedia.org/wiki/Capital_%E1%BA%9E

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nosy: +Marc Richter

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