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

[Alex Bradbury via llvm-commits]

asb wrote:

Thanks Mehdi for the review. I'll hold off on committing until James' query about recommending merging main instead is either resolved or we agree to defer to a separate discussion.

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