Requirements for scalar UDF preimage

`ScalarUDFImpl::preimage` allows defining "preimage" (range of input values) for function outputs, which can be used to rewrite binary comparison expressions.

The rewrite rules for equality (`=`) or inequality (`!=`) seem intuitive. But `f(x) <= c` would also be rewritten to `x < u` where `u` is the (exclusive) upper bound. (Here is the [code](https://github.com/apache/datafusion/pull/19722/changes#diff-3f015bb6ca85b8af8daaef1f0959e0b49ec6bcbcf675b9ea2d87d52a95893773).) I'm not sure if this holds in general. Does this assume that the function must be monotonically non-decreasing?

I was trying to see if the documentation mentions monotonicity for scalar UDFs but I couldn't find it. Let me know if I misunderstood anything!

Beginne mit dem Einstiegspunkt ScalarUDFImpl::preimage und den in pull request 19722 referenzierten Rewrite-Regeln für Gleichheit und Ungleichheit. Prüfe, ob die vorhandene Dokumentation zu skalaren UDFs Monotonie- oder Preimage-Anforderungen behandelt. Erledigt bedeutet, die erforderlichen Annahmen zu dokumentieren und zu klären, ob das <=-Rewrite allgemein gültig ist.

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