clarify group validation

Author: xabbuh

book/validation.rst

@@ -856,10 +856,10 @@ as the third argument to the ``validate()`` method::
     // If you're using the new 2.5 validation API (you probably are!)
     $errors = $validator->validate($author, null, array('registration'));
 
-    // If you're using the old 2.4 validation API
+    // If you're using the old 2.4 validation API, pass the group names as the second argument
     // $errors = $validator->validate($author, array('registration'));
 
-If no groups are specified, all constraints that belong in group ``Default``
+If no groups are specified, all constraints that belong to the group ``Default``
 will be applied.
 
 Of course, you'll usually work with validation indirectly through the form