[3.13] GH-95079: document error behaviour for some unicode C APIs (GH-95080) (#124661)

GH-95079: document error behaviour for some unicode C APIs (GH-95080)
(cherry picked from commit b79a21ea429844e84509430e636d808ea9cff244)

Co-authored-by: Max Bachmann <kontakt@maxbachmann.de>
1 file changed