Update MR pipeline page message with permission details

What does this MR do and why?

Issue #428601 (comment 1740185279)

Updated MR Pipeline prompt information to more comprehensively cover possible situations.

Final copy:

To run a merge request pipeline, the jobs in the CI/CD configuration file must be configured to run in merge request pipelines and you must have sufficient permissions in the source project.

MR acceptance checklist

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

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Create an MR and deliberately not trigger the pipeline (such as deleting the .gitlab-ci.yml file).