Update oauth2-registration.md

[T4CT1CALRANGER]

added another Info alert under the configure general information, from most recent experience, I myself had issues with 3 false rejections, due to my URL's which were all Correct & functional, not being Specific enough, adding this will help others to not have to go searching to find why they got rejected when something like one of the specific Pages, was unable to be found..

Changes

Added 2nd Alert to make sure people know that if their URL's aren't specific enough, they might get a false rejection.

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[github-actions]

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