[#1894] Update docs and example_idatasetform to reflect changes

For the 2.2.1 branch, only changes in the snippet were added, as the docs page
does not exist here

ckanext/example_idatasetform/templates/package/snippets/package_metadata_fields.html

@@ -1,9 +1,7 @@
 {% ckan_extends %}
 
-{# Remove 'free extras' from the package form. If you're using
-convert_to/from_extras() as we are with our 'custom_text' field below then
-you need to remove free extras from the form, or editing your custom field
-won't work. #}
+{# Remove 'free extras' from the package form, as we don't want to support
+   them on our example. #}
 {% block custom_fields %}
 {% endblock %}