core.xml: Wrap a Define directive sentence in a <note>

    <p>Variable names may not contain colon ":" characters, to avoid clashes

    with <directive module="mod_rewrite">RewriteMap</directive>'s syntax.</p>

    <note><title>Virtual Host scope and pitfalls</title>

      <p>While this directive is supported in virtual host context,

         the changes it makes are visible to any later configuration

         directives, beyond any enclosing virtual host.</p>

    </note>

</usage>

</directivesynopsis>