[llvm] [llvm][docs] Tweak backporting instructions a bit (PR #126519)

[Nico Weber via llvm-commits]

nico wrote:

I found the instructions not super easy to understand, so here's my attempt at improving them a bit. I've by now figured out what to do though, so I can no longer say if this is better than before 🙃

https://github.com/llvm/llvm-project/pull/126519