[clang] [OpenACC][Docs] Add a release note for Clang 21 (PR #145938)

[Aaron Ballman via cfe-commits]

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

LGTM; basically "works great for -fsyntax-only builds, less great for anything else but be patient it's a work in progress."

https://github.com/llvm/llvm-project/pull/145938