gh-113993: Don't immortalize in PyUnicode_InternInPlace; keep immortalizing in other API (#121364)

* Switch PyUnicode_InternInPlace to _PyUnicode_InternMortal, clarify docs

* Document immortality in some functions that take `const char *`

This is PyUnicode_InternFromString;
PyDict_SetItemString, PyObject_SetAttrString;
PyObject_DelAttrString; PyUnicode_InternFromString;
and the PyModule_Add convenience functions.

Always point out a non-immortalizing alternative.

* Don't immortalize user-provided attr names in _ctypes