[3.12] gh-115653: Document PyCode_GetFirstFree() (GH-115654) (#115752)

gh-115653: Document PyCode_GetFirstFree() (GH-115654)

Correct the return type of the PyCode_GetNumFree() documentation.
(cherry picked from commit 10fc4675fdb14e19f2fdd15102c6533b9f71e992)

Co-authored-by: Bogdan Romanyuk <65823030+wrongnull@users.noreply.github.com>