Improve/fix -fcatch-bottoms documentation

Old documentation suggests that -fcatch-bottoms only adds a default alternative to bottoming case expression, but that's not true. We use a very simplistic "is exhaustive" check and add default alternatives to any case expression that does not cover all constructors of the type. In case of GADTs this simple check assumes all constructors should be covered, even the ones ruled out by the type of the scrutinee.

Update the documentation to reflect this.

(Originally noticed in #17648 (closed))

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