Update NotBlank to reflect the actual validation

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

Author: DRvanR

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>`                 |