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>

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

    <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 configuration section (matching or not).

       Support for the other configuration sections was erroneously permitted by the initial

       implementation of this directive and is only preserved (with unintuitive 

       results) to maintain backward compatibility.</p>

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

    </note>

</usage>

<seealso><directive module="core">UnDefine</directive></seealso>

<seealso><directive module="core">IfDefine</directive></seealso>