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

[LLVM Continuous Integration via cfe-commits]

llvm-ci 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".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/166/builds/7360

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 2 (checkout) failure: update (failure)
git version 2.53.0
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Failed to connect to github.com port 443 after 21077 ms: Could not connect to server
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Failed to connect to github.com port 443 after 21047 ms: Could not connect to server

```

</details>

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