Managed repository template path expansions

Documents managed repository template path expansion terms, accompanying https://github.com/openmicroscopy/openmicroscopy/pull/2701. These changes are staged at http://www.openmicroscopy.org/site/support/omero5-staging/sysadmins/fs-upload-configuration.html.
Perhaps an example: "if there's a file `-1` then the next will be `-2`"