[clang] [Clang][docs] Markup corrections for attribute documentation (PR #226729)

[Reid Kleckner via cfe-commits]

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

Thanks!

I made a prototype patch that restructures the documentation as .md files which I think would help avoid bad markup issues like this in the future, but I wasn't entirely happy with it. I'll have to think more about it.

https://github.com/llvm/llvm-project/pull/226729