Improve precision of job_activity_limit_exceeded error documentation

What does this MR do and why?

Improve precision of job_activity_limit_exceeded error documentation

The current documentation states "Create too many jobs, and the pipeline fails with a job_activity_limit_exceeded error" which is misleading because it implies that defining too many jobs would fail, when actually the issue is about having too many jobs running concurrently.

Update the wording to clarify that the job_activity_limit_exceeded error occurs when too many jobs are active simultaneously, not when too many jobs are defined.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.