[Validator] Update "trim" option in the NotBlank constraint page

Author: renan-taranto

reference/constraints/NotBlank.rst

@@ -97,9 +97,10 @@ This is the message that will be shown if the value is blank.
 trim
 ~~~~
 
-**type**: ``boolean`` **default**: ``null``
+**type**: ``boolean`` **default**: ``false``
 
-If ``true`` and if the given value is a ``string``, every kind of whitespace will be
-ignored while checking if it is not blank.
+If ``true`` and if the given value is a ``string``, then the :phpfunction:`trim`
+PHP function will be used in order to ignore leading and trailing whitespace
+characters while checking if it is valid.
 
 .. include:: /reference/constraints/_payload-option.rst.inc