Update to note actions done by gitlab.com admins

What does this MR do?

Updates documentation to explain that the requirements are all set by the gitlab.com admins. GitLab users do not have ability to make these changes.

Are there any risks involved with the proposed change? What additional test coverage is introduced to offset the risk?

No

Please keep this description up-to-date with any discussion that takes place so that reviewers can understand your intent. This is especially important if they didn't participate in the discussion.

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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
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

~Documentation ~Manage