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

(cherry picked from commit bbd1f2f)

Co-authored-by: Taeknology <20297177+Taeknology@users.noreply.github.com>
Co-authored-by: Senthil Kumaran <senthil@python.org>

Author: 2 people

Doc/library/urllib.request.rst

@@ -1490,7 +1490,8 @@ 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