Add note about comparing datetime and object by reference

en/reference/basic-mapping.rst

@@ -178,6 +178,11 @@ built-in mapping types:
     a DateTime column will NOT match the datetime type that ships with
     Doctrine 2.
 
+.. note::
+
+    DateTime and Object types are compared by reference, not by value. Doctrine updates this values
+    if the reference changes and therefore behaves as if these objects are immutable value objects.
+
 .. warning::
 
     All Date types assume that you are exclusively using the default timezone