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.
``zero-pad`` might be too much of a jargon, did not get the meaning at all, would try to describe more cautiously, as this is the crucial bit to understand the behaviour of the feature.