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

(cherry picked from commit 813db24)

Co-authored-by: Pablo Galindo <[email protected]>

Author: miss-islington

Doc/library/gc.rst

@@ -137,10 +137,11 @@ The :mod:`gc` module provides the following functions:
    resulting referrers.  To get only currently live objects, call :func:`collect`
    before calling :func:`get_referrers`.
 
-   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
-   debugging.
+   .. 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
+      debugging.
 
    .. audit-event:: gc.get_referrers objs gc.get_referrers