Update to the introduction, described how 3rd and fourth arguements are...

Update to the introduction, described how 3rd and fourth arguements are required and how you can enter null for the third arguement so that you can put in validation.

manual/new/docs/en/object-relational-mapping/introduction.txt

@@ -39,3 +39,5 @@ class Email extends Doctrine_Record {
     }
 }
 </code>
+
+Note that validators / column constraints and the column length fields are optional.  The length may be omitted by using **null** for the length arguement, allowing doctrine to use a default length and permitting a fourth arguement for validation or column constraints.