gh-102591: Document that urlcleanup resets the global opener (#153562)

Co-authored-by: Senthil Kumaran <senthil@python.org>

Author: Taeknology

Doc/library/urllib.request.rst

@@ -1486,7 +1486,8 @@ some point in the future.
 .. function:: urlcleanup()
 
    Cleans up temporary files that may have been left behind by previous
-   calls to :func:`urlretrieve`.
+   calls to :func:`urlretrieve`.  It also resets the default global opener
+   installed by :func:`install_opener`.
 
 
 :mod:`!urllib.request` Restrictions