gh-128805: Clarify cElementTree deprecation notice

The deprecation notice for xml.etree.cElementTree was confusing because
it didn't explain what cElementTree is. This change clarifies that
cElementTree is an alias for ElementTree and that users should use
xml.etree.ElementTree directly instead.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kovan

Doc/library/xml.etree.elementtree.rst

@@ -17,7 +17,9 @@ 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` module, which is an alias for
+   :mod:`xml.etree.ElementTree`, is deprecated. Use :mod:`xml.etree.ElementTree`
+   directly instead.
 
 
 .. note::