Omero home for web devs 11373

Docs under web/CreateApp.txt instruct users to set $OMERO_HOME if running OMERO.web manually. See ticket https://trac.openmicroscopy.org.uk/ome/ticket/11373 and previous PR:
https://github.com/openmicroscopy/openmicroscopy/pull/1485

Also minor edit to the wording, to clarify the two setup options.

To test, check that web can be run manually from components/tools/OmeroWeb/omeroweb using python manage.py runserver and $OMERO_HOME set (as under docs).