Improve UDF usage documentation

The Designer UserGuide contains very little information about how to create a UDF, etc. Should probably add a section or refer to another document.

In the current user guide,
section 9.5 http://docs.jboss.org/teiid/designer/8.2/user-guide/en-US/html/managing-vdbs-chapter.html#udfs-in-vdb

shows FunctionModels and should be updated.