[3.11] gh-107298: Document PyMODINIT_FUNC macro (#109236) (#109948)

gh-107298: Document PyMODINIT_FUNC macro (#109236)

Document PyMODINIT_FUNC macro.

Remove links to PyAPI_FUNC() and PyAPI_DATA() macros since they are
not documented. These macros should only be used to define the Python
C API. They should not be used outside Python code base.

(cherry picked from commit d7a27e527d7e669d2e45cff80ad725978226477c)