Update serializer.rst

components/serializer.rst

@@ -704,6 +704,15 @@ always as a collection.
 .. versionadded:: 4.1
  The ``as_collection`` option was introduced in Symfony 4.1.
 
+.. tip::
+
+ XML comments are ignored by default when decoding contents, but this
+ behavior can be changed with the optional ``$ignoredNodeTypes`` argument of
+ the ``XmlEncoder`` class constructor.
+
+ .. versionadded:: 4.1
+ XML comments are ignored by default starting from Symfony 4.1.
+
 The ``YamlEncoder``
 ~~~~~~~~~~~~~~~~~~~