doc fix: hg tags defaults to current parent revision, too.
Additionally use better language for the help text for tag,
cat and manifest.
<item>
<title>#tag|escape#</title>
<link>{urlbase}{url}rev/{node|short}</link>
<description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
<pubDate>#date|rfc822date#</pubDate>
</item>