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".