Warn against using emptyOrUnsetBlockDiffSuppress

[modular-magician]

After GoogleCloudPlatform/magic-modules#6546 fixed some GKE problems, I dug into emptyOrUnsetBlockDiffSuppress a bit. I think we should recommend against using it- it's harder to understand, and is only used in two places so far.

I think what it really does is allow a user to specify an empty block when the API returns nil, which is pretty different than what O+C does. But it reads like an O+C replacement initially (and it somewhat is one). Also, I think it behaves weirdly with some values returned from the API.

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Derived from GoogleCloudPlatform/magic-modules#6595

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