[docs] [issue13828] Further improve casefold documentation

[Mark Summerfield]

Mark Summerfield added the comment:

I think the str.casefold() docs are fine as far as they go, rightly covering what it _does_ rather than _how_, yet providing a reference for the details. But what they lack is more complete information. For example I discovered this:

>>> x = "files and shuffles"
>>> x
'files and shuffles'
>>> x.casefold()
'files and shuffles'

In view of this I would add one sentence:

    In addition to lowercasing, this function also expands ligatures, for example, "fi" becomes "fi".

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nosy: +mark

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<http://bugs.python.org/issue13828>
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