Administration: Reinstate the description for the `admin_xml_ns` hook which...

Administration: Reinstate the description for the `admin_xml_ns` hook which was accidentally removed in [47771].

See #46015.
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git-svn-id: https://core.svn.wordpress.org/trunk@48149 1a063a9b-81f0-0310-95a4-ce76da25c4cd