Managed repository template path expansions (rebased from dev_5_0)

Documents managed repository template path expansion terms, accompanying https://github.com/openmicroscopy/openmicroscopy/pull/2751. These changes are staged at http://www.openmicroscopy.org/site/support/omero5.1-staging/sysadmins/fs-upload-configuration.html.

--rebased-from #836