HPCC-12007 Extend description for SERVICE keyword CONTEXT

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Merge pull request #6297 from jamienoss/issue12007-lang-ref-service-context

HPCC-12007 Extend description for SERVICE keyword CONTEXT

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>