[3.13] gh-141994: Warn of XXE vulnerability in documentation of SAX feature `xml.sax.handler.feature_external_ges` (GH-141996) (#142073)

gh-141994: Warn of XXE vulnerability in documentation of SAX feature `xml.sax.handler.feature_external_ges` (GH-141996)

Doc/library/xml.sax.handler.rst: Warn of XXE with feature_external_ges

Related to commit baa9f338971c6a13433a8232db77cd45e6b87b77
(cherry picked from commit 440bcb94560937888cd9bcb28a138acc2c6a6cbc)

Co-authored-by: Sebastian Pipping <sebastian@pipping.org>