gh-107211: No longer export internal functions (6) (#108424)

No longer export these 5 internal C API functions:

* _PyArena_AddPyObject()
* _PyArena_Free()
* _PyArena_Malloc()
* _PyArena_New()
* _Py_FatalRefcountErrorFunc()

Change comment style to "// comment" and add comment explaining why
other functions have to be exported.