Update fetching.md

The proposed changes make reference to 3 methods found in system/ee/ExpressionEngine/Service/Model/Query/Builder.php: filterGroup(), orFilterGroup(), and endFilterGroup() which have not been documented.

I stumbled upon these working on a small plugin and wanted to find a more advanced/complex method using the Model service than having to write a Query directly.

A basic example also provided for its usage.

Small change also made to remove a reference to "group_id" on a Member model example just to keep it consistent with the new Role based system.

Author: stephengalbraith

docs/development/services/model/fetching.md

@@ -46,7 +46,17 @@ The operator is optional and will default to `==` if not given:
 By default, filters will be chained as `AND` conditions. An `OR` filter can be applied by using the `orFilter()` method:
 
     ->filter('username' 'bob')
-    ->orFilter('group_id', 1)
+    ->orFilter('role_id', 1)
+
+For more complex calls, filters can be grouped by using the `filterGroup()`, `orFilterGroup()`, and `endFilterGroup()` methods:
+
+    // filter members who have less than 10 entries AND either have a primary role_id of 1 OR 2.
+    ->filter('total_entries', '<', '10')
+    ->filterGroup()
+        ->filter('role_id', '1')
+        ->orFilter('role_id', '2')
+    ->endFilterGroup()
+
 
 ### Available filters