neo_doxygen: Enhance description handling

* Document the implicit nodes.
* Prepare `DocListener` for translation of the Doxygen’s markup. #1059 must be fixed before we do anything else.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
Mes excuses. J’avoue ne pas avoir vraiment lu le code.