[llvm] [Docs] don't stylize syntax errors in code blocks (PR #212698)

[James Y Knight via llvm-commits]

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

I'd note that most of the syntax errors in LangRef are not due to pseudocode, but rather that the syntax-highlighter missing support for a bunch of LLVM IR features.

We definitely don't want to put red-boxes around any newly-introduced IR syntax in our docs.

https://github.com/llvm/llvm-project/pull/212698