Add a warning block around the get_referrers() documentation (GH-24511)

Doc/library/gc.rst

@@ -135,6 +135,7 @@ The :mod:`gc` module provides the following functions:
    resulting referrers.  To get only currently live objects, call :func:`collect`
    before calling :func:`get_referrers`.
 
+   .. warning::
       Care must be taken when using objects returned by :func:`get_referrers` because
       some of them could still be under construction and hence in a temporarily
       invalid state. Avoid using :func:`get_referrers` for any purpose other than