HPCC-9620 Document ecl roxie --unused-files

Signed-off-by: JamesDeFabia <James.Defabia@LexisNexis.com>
Merge pull request #4629 from JamesDeFabia/hpcc9620_Document_ecl_roxie_unused-files

HPCC-9620 Document ecl roxie --unused-files

Reviewed-By: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>