Document overriding XCom.clear for data lifecycle management (apa…

…che#17589)

This was added in apache#17405 but was not documented

docs/apache-airflow/concepts/xcoms.rst

@@ -47,3 +47,5 @@ The XCom system has interchangeable backends, and you can set which backend is b
 If you want to implement your own backend, you should subclass :class:`~airflow.models.xcom.BaseXCom`, and override the ``serialize_value`` and ``deserialize_value`` methods.
 
 There is also an ``orm_deserialize_value`` method that is called whenever the XCom objects are rendered for UI or reporting purposes; if you have large or expensive-to-retrieve values in your XComs, you should override this method to avoid calling that code (and instead return a lighter, incomplete representation) so the UI remains responsive.
+
+You can also override the ``clear`` method and use it when clearing results for given dags and tasks. This allows the custom XCom backend process the data lifecycle easier.