[clang] [clang][docs] Clarify [[unlikely]] example in compound statement (PR #186590)

[via cfe-commits]

Sirraide wrote:

> LLVM Buildbot has detected a new failure on builder `flang-x86_64-windows` running on `minipc-ryzen-win` while building `clang` at step 2 "checkout".

(this is unrelated; you can ignore that)

https://github.com/llvm/llvm-project/pull/186590