HPCC-10226 Document change to EXPIRE semantics

Signed-off-by: JamesDeFabia <James.Defabia@LexisNexis.com>
Merge pull request #5000 from JamesDeFabia/10226DocumentChangeToExpireSemantics

HPCC-10226 Document change to EXPIRE semantics

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>