[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:

@joker-eph: Thanks, I've adopted that change.

@jyknight: Given none of our current documentation discusses introducing merge commits (even if they disappear before hitting the main branch) I think this would be more complex to explain. For me, I'm only really used to this rebase-focused workflow so I'm not actually sure how GitHub would handle your suggestion in its UI - both when showing changes for review and in terms of allowing "squash and merge" to commit. I'd love to learn more though. Is there a particular advantage to doing it as you suggest?

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