Document the Custom option of the generated value annotation

docs/en/reference/basic-mapping.rst

@@ -363,6 +363,8 @@ Here is the list of possible generation strategies:
    thus generated) by your code. The assignment must take place before
    a new entity is passed to ``EntityManager#persist``. NONE is the
    same as leaving off the @GeneratedValue entirely.
+-  ``CUSTOM``: With this option, you can use the ``@CustomIdGenerator`` annotation.
+   It will allow you to pass a :ref:`class of your own to generate the identifiers <_annref_customidgenerator>`
 
 Sequence Generator
 ^^^^^^^^^^^^^^^^^^