[llvm] [docs] Make rebase advice more explicit for cases where pre-commit CI is failing (PR #76704)

[Alex Bradbury via llvm-commits]

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@@ -105,7 +105,9 @@ branch that's the root of the Pull Request during the review. This action will
 make the context of the old changes and comments harder to find and read.
 
 Sometimes, a rebase might be needed to update your branch with a fix for a test
-or in some dependent code.
+or in some dependent code. This is especially encouraged if it turns out that
+the upstream base commit used for your branch had test failures, meaning the
+pre-commit CI results are not useful.
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asb wrote:

How about "not useful" => "misleading"? The big issue with failures is that you see that red cross and have to click through for more details to see if it's a "real" problem (an issue with the PR author's patch) or not.

https://github.com/llvm/llvm-project/pull/76704