add caution for datetime object

Author: gilles-g

serializer.rst

@@ -164,6 +164,12 @@ properties and setters (``setXxx()``) to change properties:
     introduced in Symfony 3.4. In previous Symfony versions only getters (``getXxx()``)
     and issers (``isXxx()``) were supported.
 
+.. caution::
+
+    Be aware that the use the ``GetSetMethodNormalizer``, without the ``DateTimeZoneNormalizer`` for normalize object
+    ``DateTimeInterface``, increase the memory use of PHP. Because the ``Datetime`` class has getters, the
+    ``GetSetMethodNormalizer`` will just normalize the ``Datetime`` object.
+
 .. _serializer-using-serialization-groups-annotations:
 
 Using Serialization Groups Annotations