[Docs] CodingStandards: for_each is discouraged

Summary:
As per disscussion in D83351, using `for_each` is potentially confusing,
at least in regards to inconsistent style (there's less than 100 `for_each`
usages in LLVM, but ~100.000 `for` range-based loops

Therefore, it should be avoided.

Reviewers: dblaikie, nickdesaulniers

Reviewed By: dblaikie, nickdesaulniers

Subscribers: hubert.reinterpretcast, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83431