[3.12] gh-90241: Clarify documentation for PyUnicode_FSConverter and PyUnicode_FSDecoder (GH-128451) (GH-128543)

(cherry picked from commit 657d7b77e5c69967e9c0000b986fa4872d13958c)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>