[llvm] [nfc] Add doc comment for `canReturn` in Analysis/CFG.h (PR #135862)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Apr 15, 5:41 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/135862).


https://github.com/llvm/llvm-project/pull/135862