Fix nuxeo-retention-technical-overview page about ecm:isUnderRetentionOrLegalHold

The doc mentions in https://doc.nuxeo.com/nxdoc/nuxeo-retention-technical-overview/ that ecm:isUnderRetentionOrLegalHold is a "Dedicated Low-Level Fields" but it is not. There is no such field persisted in the db backend.

It is a convenient "composite" system property returned when serializing a doc with the following semantics:

 Calendar retainUntil;
        return hasLegalHold()
                || (((retainUntil = getRetainUntil()) != null) && Calendar.getInstance().before(retainUntil));

One could think it is possible to run such query:

SELECT * FROM Document WHERE ecm:isUnderRetentionOrLegalHold = 1