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.
I am afraid the ``Path naming constraints`` should be rewritten with following in mind:
  - atm it is too verbose, start bullet points with WHAT and then WHY and WHO only, e.g. ``components preceding // are owned by the root user`` should come to the start of the bullet point, the ``server ensures it…`` part comes later in the bullet point
  - make a bullet point out of the ``Furthermore, due to the availability of the expansion terms enumerated
 +below, it is advisable to avoid other uses of the ":literal:`%`"
 +character in path components.`` sentence and shorten it in the process
Try to avoid brackets.