Replace OMERO_HOME environment variable by OMERO_PREFIX
Replace OMERO_HOME by OMERO_SOURCE_PREFIX when pointing at source code
Add warning about setting the OMERO_HOME environment variable
Improve readability of paragraphs referring to OMERO_PREFIX
Restore usage of OMERO_HOME variable in OMERO.grid page
Expand exported variable and removed unclear paragraph
Merge pull request #320 from sbesson/rebased/develop/10603_omerohome_envvar

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