document new // separator in omero.fs.repo.path

reflects changes arising from fixing
http://trac.openmicroscopy.org.uk/ome/ticket/12160
warn that in-place ln_s import can be slower

see https://trac.openmicroscopy.org.uk/ome/ticket/12217
adjust for permitting no root-owned directories
Merge pull request #780 from mtbc/in-place-warning

warn that in-place ln_s can be slower for HCS
Merge pull request #778 from mtbc/template-path-ownership

document new // separator in omero.fs.repo.path