parameter descriptions for XML/GML readers and writers

Change-Id: Ie80f91f5786984cf52a63dcde66cd7247d7d6b29
Reviewed-on: http://gerrit.esdi-community.eu/3117
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>