[3.13] gh-141004: Document `Py_UNICODE_{HIGH, LOW}_SURROGATE` functions (GH-141019) (GH-141026)

gh-141004: Document `Py_UNICODE_{HIGH, LOW}_SURROGATE` functions (GH-141019)
(cherry picked from commit bfe54810c408ff066591d1af0411b1d9c10084b1)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>