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> http://hg.python.org/cpython/rev/0b5ce36a7a24
> changeset:  74515:0b5ce36a7a24
> +  Casefolding is similar to lowercasing but more aggressive because it is
> +  intended to remove all case distinctions in a string. For example, the German
> +  lowercase letter ``'ß'`` is equivalent to ``"ss"``. Since it is already
> +  lowercase, :meth:`lower` would do nothing to ``'ß'``; :meth:`casefold`
> +  converts it to ``"ss"``.
Perhaps add the recommendation to canonicalize as well.
A complete, but possibly too long, try is below:
Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. For example, the German lowercase letter ``'ß'`` is equivalent to ``"ss"``. Since it is already lowercase, :meth:`lower` would do nothing to ``'ß'``; :meth:`casefold` converts it to ``"ss"``. Note that most case-insensitive matches should also match compatibility equivalent characters. 
The casefolding algorithm is described in section 3.13 of the Unicode Standard. Per D146, a compatibility caseless match can be achieved by
 from unicodedata import normalize
 def caseless_compat(string):
 nfd_string = normalize("NFD", string)
 nfkd1_string = normalize("NFKD", nfd_string.casefold())
 return normalize("NFKD", nfkd1_string.casefold())