Do not expose OMERO_HOME in the doc (see #10603) (rebased onto develop)



This is the same as gh-310 but rebased onto develop.

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`OMERO_HOME` is an internal variable used in `bin/omero` and should not be set unless people really know what they are doing.

This PR replaces all occurrences of `OMERO_HOME` by either
- `OMERO_PREFIX` when pointing at a OMERO.server instance
- `OMERO_SOURCE_PREFIX` when pointing at the source code root directory