Fix stale documentation on {ForeignKeyed,}DesignatedContact

Noticed that this is a bit out-out-date ever since these were
split into two types versus using the converters.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150352102

java/google/registry/model/domain/ForeignKeyedDesignatedContact.java

@@ -19,8 +19,11 @@ import javax.xml.bind.annotation.XmlAttribute;
 import javax.xml.bind.annotation.XmlValue;
 
 /**
- * EPP-compatible version of XML type for contact identifiers associated with a domain, which can
- * be converted to a storable {@link DesignatedContact}.
+ * EPP-XML-serializable equivalent of {@link DesignatedContact}.
+ *
+ * <p>This type is used on the wire for EPP XML, where only the contact ID (foreign key) is exposed.
+ * This is converted to and from the persisted type, {@link DesignatedContact}, which stores the
+ * Datastore key instead of the foreign key.
  *
  * @see <a href="http://tools.ietf.org/html/rfc5731#section-2.2">
  *     RFC 5731 - EPP Domain Name Mapping - Contact and Client Identifiers</a>