[docs] validate invitation on registration only

Author: nnscr

Resources/doc/adding_invitation_registration.md

@@ -100,7 +100,7 @@ class User extends \FOS\UserBundle\Entity\User
     /**
      * @ORM\OneToOne(targetEntity="Invitation", inversedBy="user")
      * @ORM\JoinColumn(referencedColumnName="code")
-     * @Assert\NotNull(message="Your invitation is wrong")
+     * @Assert\NotNull(message="Your invitation is wrong", groups={"Registration"})
      */
     protected $invitation;