docs: add description

Author: kenjis

user_guide_src/source/models/model.rst

@@ -145,7 +145,7 @@ This requires either a DATETIME or INTEGER field in the database as per the mode
 `$dateFormat`_ setting. The default field name is ``deleted_at`` however this name can be
 configured to any name of your choice by using `$deletedField`_ property.
 
-.. important:: The ``deleted_at`` field must be nullable.
+.. important:: The ``deleted_at`` field in the database must be nullable.
 
 $allowedFields
 --------------