document graph traversal reimplementation for developers

Documents the changes to server code brought by https://github.com/openmicroscopy/openmicroscopy/pull/3228; some of the links may remain broken until it is merged. Mostly staged at http://www.openmicroscopy.org/site/support/omero5.1-staging/developers/Server/ObjectGraphs.html.
--no-rebase
It does: The imitation of the old `omero.cmd.Delete` covers the expected bases but is not comprehensive and one of the small differences in the behavior of `Delete2` may break something upon legacy third-party scripts are relying. So, at least for a transition period, I think it makes sense to make the old implementation accessible for those who need time to migrate.