synopsis.xsl: do not render two times the same

              directive HTML if more than one
              directive share the same name.

This has happened when mod_md.xml was introduced,
and the following directives shared the same name:
* ManagedDomain
* <ManagedDomain>

With the current code each time that a node needs
to be rendered it will emit a duplicate, ending up
in the above example with 4 sections rather than two.

Uniqueness of sections will be ensured by the HTML
elements ids, to avoid errors before committing for
example (accidental duplicates, etc..).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1805189 13f79535-47bb-0310-9956-ffa450edef68