[llvm] [VPlan] Add additional guiding principles to docs. (PR #85688)

[Nikolay Panchenko via llvm-commits]

https://github.com/npanchen commented:

That you Florian! Not sure if you planned to use that PR for discussion purposes, but I left few comments to get better clarity.

https://github.com/llvm/llvm-project/pull/85688