Fix issues found when documenting current expression incompatibilities

[andygrove]

Describe the bug

Some issues were discovered when reviewing #4067. This issue is a follow up for fixing these.

• CometSum incorrectly states that it is incompatible in ANSI mode (https://github.com/apache/datafusion-comet/pull/4067/changes#r3140308388)
• CometStringRepeat has an incompatibility that we do not fall back for (see https://github.com/apache/datafusion-comet/pull/4067/changes#r3140328947)
• CometFromUnixTime incorrect reports Incompatible instead of Unsupported for unsupported formats

Steps to reproduce

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Expected behavior

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Additional context

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