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

[Alex Bradbury via llvm-commits]

asb wrote:

I've started a discourse discussion [here](https://discourse.llvm.org/t/use-of-merge-commits-in-prs-under-review/76147) to try to move this forward, but as noted I don't think this clarification/refinement of existing recommended practice necessarily needs to block on the potential decision about changing that recommended practice.

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