it appears the docs for .things requirements are missing
Sadly there is no documentation for things yet as the page docs.openhab.org is still pretty young and volunteers by the community are giving their best to fill the holes.
General information on things can be found here: http://docs.openhab.org/concepts/things.html
Examples for every native OH2 binding can be found in their respecitive docs, e.g. here, here or here