<xsl:for-each select="umbraco.library:GetXmlAll()//MyNewsItemDocType">
   <xsl:sort select="@updateDate" order="descending"/>
    <!-- display news items here -->
</xsl:for-each>

This will iterate all news item doctypes from across the site, sorted by last updated, with most recent first.

If there is a "News Date" field on your doctype you can obviously use that instead of @updateDate.

Here is a slight variation to only display 5 most recent items (useful on home pages for example):

<xsl:for-each select="umbraco.library:GetXmlAll()//MyNewsItemDocType">
    <xsl:sort select="@updateDate" order="descending"/>
    <xsl:if test="position() &lt; 5">
        <!-- display news items here -->
    </xsl:if>
</xsl:for-each>

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