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.
What do you mean by natural number here? I know the (disputed) mathematical meaning but are you implying decimal. It doesn't imply sequential or consecutive though that's what I understand this term to be. cf the `%subdir%` explanation below where slightly different terminology is used for a similar idea.