HPCC-8865 Document Escape Character in SprayVariable ECL library function

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4099 from richardkchapman/minordocs2a

HPCC-8865 Document Escape Character in SprayVariable ECL library functio...

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>