docs: documents Date validator strict mode

Author: weierophinney

docs/book/validators/date.md

@@ -33,3 +33,24 @@ $validator = new Zend\Validator\Date(['format' => 'Y']);
 $validator->isValid('2010'); // returns true
 $validator->isValid('May');  // returns false
 ```
+
+## Strict mode
+
+- **Since 2.13.0**
+
+By default, `Zend\Validator\Date` only validates that it can convert the
+provided value to a valid `DateTime` value.
+
+If you want to require that the date is specified in a specific format, you can
+provide both the [date format](#specifying-a-date-format) and the `strict`
+options. In such a scenario, the value must both be covertable to a `DateTime`
+value **and** be in the same format as provided to the validator. (Generally,
+this will mean the value must be a string.)
+
+```php
+$validator = new Zend\Validator\Date(['format' => 'Y-m-d', 'strict' => true]);
+
+$validator->isValid('2010-10-10'); // returns true
+$validator->isValid(new DateTime('2010-10-10)); // returns false; value is not a string
+$validator->isValid('2010.10.10'); // returns false; format differs
+```