[llvm] [docs] Document `-mtriple` option in opt command guide (NFC) (PR #213942)

[Reid Kleckner via llvm-commits]

https://github.com/rnk approved this pull request.

The logic is quote convoluted, hard to follow, and duplicated with llc: https://github.com/llvm/llvm-project/blob/2f9775ad49bf8eb0d4a54592d40652df3e4f38dd/llvm/tools/opt/optdriver.cpp#L568

My reading, without testing, is that it overrides, and we should document what it does today before we consider changing it.

https://github.com/llvm/llvm-project/pull/213942