Bug 36009: Document koha-worker --queue elastic_index

Bug 36009: Document koha-worker --queue elastic_index

To test:
- Copy the koha-worker script to the standard location:
  $ sudo cp debian/scripts/koha-worker /usr/sbin/koha-worker
- Check "sudo koha-worker --help" and verify elastic_index is
  mentioned in the list of "current queues"
- See https://wiki.koha-community.org/wiki/Testing_man_pages for
  how to check the "man" page. There should be a new paragraph
  about "Current queues" under "--queue"

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>