Update permission descriptions for documentation and FE

What does this MR do and why?

Minor improvements to permission descriptions that displays on docs and FE.

• Updates to: https://docs.gitlab.com/ee/user/custom_roles/abilities.html + FE
• Includes
  • Updated descriptions on permissions
  • Remove duplicate headers (groups and projects)
  • Consolidate headers between source code management and code review
  • Regenerate docs

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