Update scan execution policy scheduled pipeline documentation

We added a new worker in MR !147691 (merged) to process the scan_execution_policy schedule pipeline creation. We also added an application setting for the concurrency limit in MR !152855 (merged). The MR !158636 (closed) will use this new limit to control the concurrency of Ci::Build jobs.

We should update the scan_execution_policy schedule documentation to reflect these changes.