Update wording for policy access from SPEP

What does this MR do and why?

Although we mention this fact in the Requirements section, the "Check policy access" point is insufficiently described and it's not clear that it's not enough to just enable the checkbox on the security policy project. The pipeline execution policy CI/CD configuration also needs to be stored in the security policy project.

References

• https://docs.gitlab.com/user/application_security/policies/scheduled_pipeline_execution_policies/#troubleshooting

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