[3.15] gh-128805: Clarify the deprecation notice of xml.etree.cElementTree (GH-156779) (#156883)

gh-128805: Clarify the deprecation notice of xml.etree.cElementTree (GH-156779)

It was read as deprecating xml.etree.ElementTree itself, because the page
does not mention cElementTree anywhere else.
(cherry picked from commit 50ed1ec)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: miss-islington

Doc/library/xml.etree.elementtree.rst

@@ -15,7 +15,7 @@ for parsing and creating XML data.
    This module will use a fast implementation whenever available.
 
 .. deprecated:: 3.3
-   The :mod:`!xml.etree.cElementTree` module is deprecated.
+   The :mod:`!xml.etree.cElementTree` alias of this module is deprecated.
 
 
 .. note::