Clarify requirements for custom metrics in docs

What does this MR do?

Before	After

The custom metrics form returns 404 if there isn't a cluster with environment scope * and Prometheus installed or a manually configured Prometheus available. This is unfortunately not clear in the docs and on the UI. This MR adds help text next to the custom metrics section. This MR also modifies the docs to mention the above.

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31286

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/62234

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