[ckan#1894] Update docs and example_idatasetform to reflect changes

I kept the example of removing the default key/value extras, as this is
what most people would want to do anyway, but mentioning that you have
the option of combining them both on CKAN >= 2.2.1

Conflicts:
	doc/extensions/adding-custom-fields.rst

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 %}