HPCC-8864 - Add comment/code clearup

Add comment to explain psuedo filename passed to createKeyIndex

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4142 from jakesmith/hpcc-8864

HPCC-8864 - Add comment/code clearup

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>