HPCC-12596 Improve eclcc help text

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6651 from ghalliday/issue12596

HPCC-12596 Improve eclcc help text

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>