Update NotBlank to reflect the actual validation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | n/a

As can be seen in the [validator][1] class values that are false also lead to a violation. This was not documented, which we discovered when using this constraint :wink:

[1]: https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Validator/Constraints/NotBlankValidator.php#L34