Draft: Document exceptional case and when it is allowed

What does this MR do and why?

Document exceptional case and when it is allowed

In this case we allow a CHECK constraint for mixed cell-local and organizational data.

Related: !156380 (comment 1977732140)

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https://main-ee-158205.docs.gitlab-review.app/ee/development/database/multiple_databases.html#the-sharding-key-must-be-non-nullable

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