Document the behavior

Author: vstinner

Doc/c-api/refcounting.rst

@@ -34,6 +34,9 @@ of Python objects.
 
    Set the object *o* reference counter to *refcnt*.
 
+   On ref:`Python build with Free Threading <free-threading-buid>`, if *refcnt*
+   is larger than ``UINT32_MAX``, the object is made :term:`immortal`.
+
    This function has no effect on :term:`immortal` objects.
 
    .. versionadded:: 3.9

Doc/using/configure.rst

@@ -287,6 +287,8 @@ General Options
 
    .. versionadded:: 3.11
 
+.. _free-threading-buid:
+
 .. option:: --disable-gil
 
    Enables **experimental** support for running Python without the