[docs] [issue32124] Document functions safe to be called before Py_Initialize()

Serhiy Storchaka storchaka+cpython@gmail.com added the comment:

I'm quite sure that calling PyMem_SetAllocator() or PyObject_SetArenaAllocator() after Py_Initialize() will quickly crash.

Then document this explicitly like for other functions that *should* be called before Py_Initialize() if called at all.

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