Clarify behavior of compare

What does this MR do and why?

Clarification of compare behavior. I was wondering why cherry picks aren't handled corretcly in Gitlab. I didn't find anything related in the docs, but I found this issue explaining the intent, implementation and limitations of compare by @DouweM Link. Having this in the documentation would have saved me a few hours of research.

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Related to Compare revisions "straight mode" logic is back... (#382619 - closed)