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

[Mehdi Amini via llvm-commits]

================
@@ -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.
----------------
joker-eph wrote:

It may be phrased a bit strongly: I often see test failures in pre-commit CI in other projects like  while modifying MLIR: as long as MLIR and Flang tests are passing, the pre-commit CI results aren't "useless".
I would say only when there is a build failure and the tests can't run that the results aren't useful anymore.

That said the advice isn't bad for beginner.

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