added note on reserved keywords to Groovy help

Change-Id: I9dbe009cb694ed68286e1a5f034e030d0b474d9c
Reviewed-on: http://gerrit.esdi-community.eu/3166
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>