bug #4615 Update NotBlank to reflect the actual validation (DRvanR)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4615).

Discussion
----------

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 😉

[1]: https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Validator/Constraints/NotBlankValidator.php#L34

Commits
-------

b11ad2e Update NotBlank to reflect the actual validation

reference/constraints/NotBlank.rst

@@ -1,9 +1,9 @@
 NotBlank
 ========
 
-Validates that a value is not blank, defined as not equal to a blank string
-and also not equal to ``null``. To force that a value is simply not equal
-to ``null``, see the :doc:`/reference/constraints/NotNull` constraint.
+Validates that a value is not blank, defined as not strictly ``false``, not equal to a blank
+string and also not equal to ``null``. To force that a value is simply not equal to
+``null``, see the :doc:`/reference/constraints/NotNull` constraint.
 
 +----------------+------------------------------------------------------------------------+
 | Applies to     | :ref:`property or method <validation-property-target>`                 |