Vulnerabilities API now supports pipeline_id

What does this MR do?

Updates Vulnerabilities API documentation.

Related to EE Merge Request https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14376

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content