[SNC-590] Update usage text for "policy-base-url" on Server

[jbialy]

Checklist

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• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have checked for similar issues and haven't found anything relevant.
• [x] This is not a security issue (which should be reported here: https://circleci.com/security/)
• [x] I have read Contribution Guidelines.

Internal Checklist

• [x] I am requesting a review from my own team as well as the owning team
• [ ] I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

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• Adjusted the parameter usage text to specifically call out the requirement for Server customers.

Rationale

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Policy management is available to Server customer however, they may not be aware that in order to use the cli setting the policy-base-url flag is required.

Considerations

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Leave helper text as is and only relay on the update to the public docs around cli usage in Server. See PR https://github.com/circleci/circleci-docs/pull/8588.