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

Doc/c-api/unicode.rst

@@ -1490,18 +1490,43 @@ They all return ``NULL`` or ``-1`` if an exception occurs.
    existing interned string that is the same as :c:expr:`*p_unicode`, it sets :c:expr:`*p_unicode` to
    it (releasing the reference to the old string object and creating a new
    :term:`strong reference` to the interned string object), otherwise it leaves
-   :c:expr:`*p_unicode` alone and interns it (creating a new :term:`strong reference`).
+   :c:expr:`*p_unicode` alone and interns it.
+
    (Clarification: even though there is a lot of talk about references, think
-   of this function as reference-neutral; you own the object after the call
-   if and only if you owned it before the call.)
+   of this function as reference-neutral. You must own the object you pass in;
+   after the call you no longer own the passed-in reference, but you newly own
+   the result.)
+
+   This function never raises an exception.
+   On error, it leaves its argument unchanged without interning it.
+
+   Instances of subclasses of :py:class:`str` may not be interned, that is,
+   :c:expr:`PyUnicode_CheckExact(*p_unicode)` must be true. If it is not,
+   then -- as with any other error -- the argument is left unchanged.
+
+   Note that interned strings are not “immortal”.
+   You must keep a reference to the result to benefit from interning.
 
 
 .. c:function:: PyObject* PyUnicode_InternFromString(const char *str)
 
    A combination of :c:func:`PyUnicode_FromString` and
-   :c:func:`PyUnicode_InternInPlace`, returning either a new Unicode string
-   object that has been interned, or a new ("owned") reference to an earlier
-   interned string object with the same value.
+   :c:func:`PyUnicode_InternInPlace`, meant for statically allocated strings.
+
+   Return a new ("owned") reference to either a new Unicode string object
+   that has been interned, or an earlier interned string object with the
+   same value.
+
+   Python may keep a reference to the result, or make it :term:`immortal`,
+   preventing it from being garbage-collected promptly.
+   For interning an unbounded number of different strings, such as ones coming
+   from user input, prefer calling :c:func:`PyUnicode_FromString` and
+   :c:func:`PyUnicode_InternInPlace` directly.
+
+   .. impl-detail::
+
+      Strings interned this way are made :term:`immortal`.
+
 
 PyUnicodeWriter
 ^^^^^^^^^^^^^^^