[Selection plugin Documentation] Improve page "DynamicDropdown configuration"

This is an improvement about page http://selection.forge.onehippo.org/configuration/dynamicdropdown.html

In 3rd paragraph at the top, it reads: "In the document namespace, editor section, your field can have property valuelist.provider to be pointing at a value list provider CMS service. When not filled in, it defaults to service.valuelist.default which is the DocumentValueListProvider."

The above statement is not true. Having the property 'valuelist.provider' in the editor section has no effect. What's supported instead is adding the property 'valuelistProvider' under cluster.options node.