HPCC-9064 Document the changes to the ROLLUP semantics

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4256 from ghalliday/issue9064

HPCC-9064 Document the changes to the ROLLUP semantics

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>