Add doc for the checkHost email constraint (#1414)

Author: Dinduks

reference/constraints/Email.rst

@@ -9,6 +9,7 @@ cast to a string before being validated.
 +----------------+---------------------------------------------------------------------+
 | Options        | - `message`_                                                        |
 |                | - `checkMX`_                                                        |
+|                | - `checkHost`_                                                        |
 +----------------+---------------------------------------------------------------------+
 | Class          | :class:`Symfony\\Component\\Validator\\Constraints\\Email`          |
 +----------------+---------------------------------------------------------------------+
@@ -46,17 +47,17 @@ Basic Usage
                 </property>
             </class>
         </constraint-mapping>
-        
+
     .. code-block:: php-annotations
 
         // src/Acme/BlogBundle/Entity/Author.php
         namespace Acme\BlogBundle\Entity;
-        
+
         use Symfony\Component\Validator\Constraints as Assert;
 
         class Author
         {
-            /** 
+            /**
              * @Assert\Email(
              *     message = "The email '{{ value }}' is not a valid email.",
              *     checkMX = true
@@ -83,4 +84,14 @@ checkMX
 If true, then the `checkdnsrr`_ PHP function will be used to check the validity
 of the MX record of the host of the given email.
 
-.. _`checkdnsrr`: http://www.php.net/manual/en/function.checkdnsrr.php
+.. _`checkdnsrr`: http://www.php.net/manual/en/function.checkdnsrr.php
+
+checkHost
+~~~~~~~~~
+
+**type**: ``Boolean`` **default**: ``false``
+
+If true, then the `checkdnsrr`_ PHP function will be used to check the validity
+of the MX *or* the A *or* the AAAA record of the host of the given email.
+
+.. _`checkdnsrr`: http://www.php.net/manual/en/function.checkdnsrr.php