fix: document actual behavior of add_continuous_aggregate_policy when using if_not_exists

[dtext]

Description

The documentation says to enable if_not_exists if I don't want it to error when the policy does not exist, but when adding a policy, it should be the other way around.

Normally, it errors when the policy already exists, and when specifying if_not_exists, I want it to warn instead.

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• [ ] Is the content technically accurate?
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Documentation team review checklist

• [ ] Is the content free from typos?
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