Upgrade to 16.04 LTS not offered

According to Ubuntu documentation a straightforward upgrade from 14.04 LTS to 16.04 LTS should be possible.
However, our Trusty Tahr does not offer such upgrade.
It does not offer any upgrade until following change in systems settings (Software & Updates) is made: Notify me of new Ubuntu version - For any new version;
the choice "for long-term support versions" is not sufficient enough for Trasty to detect the availability of Xenial Xerus.
User expectation is "for long-term support versions" enables Trusty to detect availibility of Xenial upgrade.

Even if to enable "Notify me of new Ubuntu version - For any new version" just 15.10 is offered, but not Xenial.