docs: make description more detailed

Author: kenjis

user_guide_src/source/models/model.rst

@@ -659,9 +659,14 @@ prior to saving to the database with the ``insert()``, ``update()``, or ``save()
 .. important:: When you update data, by default, the validation in the model class only
     validates provided fields. This is to avoid validation errors when updating only some fields.
 
-    But this means ``required*`` rules do not work as expected when updating.
-    If you want to check required fields, you can change the behavior by configuration.
-    See :ref:`clean-validation-rules` for details.
+    However, this means that not all validation rules you set will be checked
+    during updates. Thus, incomplete data may pass the validation.
+
+    For example, ``required*`` rules or ``is_unique`` rule that require the
+    values of other fields may not work as expected.
+
+    To avoid such glitches, this behavior can be changed by configuration. See
+    :ref:`clean-validation-rules` for details.
 
 Setting Validation Rules
 ------------------------