Update item query guide

Author: Flo0807

Diff for: guides/live_resource/item-query.md

@@ -25,10 +25,16 @@ use Backpex.LiveResource,
     query
     |> where([post], post.published)
   end
+
+  def item_query(query, _live_action, _assigns) do
+    query
+  end
 ```
 
 The example above will filter all posts by a published boolean on `index` view. We also made use of the named binding. It's always the name of the provided schema in `snake_case`. It is recommended to build your `item_query` on top of the incoming query. Otherwise you will likely get binding errors.
 
+Make sure to always cover all possible cases or add a fallback `item_query/3` function that just returns the query.
+
 > #### Important {: .info}
 >
 > Note that it is not possible to use an anonymous function for `item_query` configuration. You must refer to a public function defined within a module.