Add notes to sys.intern and PyUnicode_InternInPlace about immortal strings

Author: HernaniSamuel

Doc/c-api/unicode.rst

@@ -1753,6 +1753,8 @@ They all return ``NULL`` or ``-1`` if an exception occurs.
    Note that interned strings are not “immortal”.
    You must keep a reference to the result to benefit from interning.
 
+   .. note::
+      In the free-threaded build, all interned strings are :term:`immortal`.
 
 .. c:function:: PyObject* PyUnicode_InternFromString(const char *str)
 

Doc/library/sys.rst

@@ -1325,6 +1325,9 @@ always available. Unless explicitly noted otherwise, all variables are read-only
    Interned strings are not :term:`immortal`; you must keep a reference to the
    return value of :func:`intern` around to benefit from it.
 
+   .. note::
+
+      In the free-threaded build, all interned strings are :term:`immortal`.
 
 .. function:: _is_gil_enabled()