[docs] [issue13828] Further improve casefold documentation

[Raymond Hettinger]

Raymond Hettinger added the comment:

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

+1 I would have found that sentence to be helpful.

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nosy: +rhettinger

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