Clearify behaviour of Blank and NotBlank validator

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets |

Author: David Fuhr

reference/constraints/Blank.rst

@@ -3,8 +3,16 @@ Blank
 
 Validates that a value is blank, defined as equal to a blank string or equal
 to ``null``. To force that a value strictly be equal to ``null``, see the
-:doc:`/reference/constraints/IsNull` constraint. To force that a value is
-*not* blank, see :doc:`/reference/constraints/NotBlank`.
+:doc:`/reference/constraints/IsNull` constraint.
+
+
+.. tip::
+
+    To force that a value is *not* blank, see
+    :doc:`/reference/constraints/NotBlank`. But be careful as ``NotBlank`` is
+    *not* strictly the opposite of ``Blank``. Emtpy array and strictly
+    ``false`` are considered blank by the ``NotBlank`` validator but not by the
+    ``Blank`` validator.
 
 +----------------+---------------------------------------------------------------------+
 | Applies to     | :ref:`property or method <validation-property-target>`              |

reference/constraints/NotBlank.rst

@@ -2,8 +2,8 @@ NotBlank
 ========
 
 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
+not equal to a blank string, not an empty array and also not equal to ``null``.
+To force that a value is simply not equal to ``null``, see the
 :doc:`/reference/constraints/NotNull` constraint.
 
 +----------------+------------------------------------------------------------------------+