doc: Updates GlobalClusterConfig doc about self-managed sharding clusters

[lantoli]

Updates GlobalClusterConfig doc about self-managed sharding clusters

Jira ticket: CLOUDP-258466

Please include a summary of the fix/feature/change, including any relevant motivation and context.

Link to any related issue(s):

Type of change:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Manual QA performed:

• [ ] cfn invoke for each of CRUDL/cfn test
• [ ] Updated resource in example
• [ ] Published to AWS private registry
• [ ] Used the template in example to create and update a stack in AWS
• [ ] Deleted stack to ensure resources are deleted
• [ ] Created multiple resources in same stack
• [ ] Validated in Atlas UI
• [ ] Included screenshots

Required Checklist:

• [x] I have signed the MongoDB CLA
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• [x] I have added any necessary documentation (if appropriate)
• [x] I have run make fmt and formatted my code
• [x] For CFN Resources: I have released by changes in the private registry and proved by change works in Atlas

Further comments