Add documentation for the retention flag

In the 9.3 release notes we mentioned:
"A new method has been added to the CoreSession so as to set a flag "ecm:isRetentionActive" on a document in order to specify it is in retention:
session.setRetentionActive(doc.getRef(), true);
An event "retentionActiveChanged" is sent and an entry is logged in the audit trail when the method is called."

This needs to be added to the documentation.