minor #1400 [docs] validate invitation on registration only (nnscr)

This PR was squashed before being merged into the 2.0.x-dev branch (closes #1400).

Discussion
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[docs] validate invitation on registration only

See issue #1399, just a minor change in the documentation

Commits
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a05e866 [docs] validate invitation on registration only

Author: stof

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;