Revert "allow xml in module description"

This reverts commit a2c9587165.

Somehow the OCaml XML/DTD parser doesn't like it if the description contains CDATA...

conf/modules/nav_survey_polygon.xml

@@ -15,7 +15,7 @@ In the flight plan, the survey blocks call init and run functions:
   - min_rad       minimal radius when navigating
   - altitude      the altitude that must be reached before the flyover starts
 2. Run the survey with nav_survey_polygon_run()
-
+<!--
 Block example:
 @verbatim
 <block name="Poly Survey" strip_button="Poly Survey">
@@ -23,6 +23,7 @@ Block example:
   <call fun="nav_survey_polygon_run()"/>
 </block>
 @endverbatim
+-->
     </description>
   </doc>
   <header>