[clang] Document the [[clang::annotate]] attribute (PR #203303)

[Balázs Benics via cfe-commits]

steakhal wrote:

@AaronBallman  I figured `clang::annotate` is translated to LLVM IR, thus is not really a good fit for clang tooling annotations.
I only have vague memories of this rumour, so correct me if this is not the case.

https://github.com/llvm/llvm-project/pull/203303