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

[Mircea Trofin via llvm-commits]

mtrofin wrote:

* **#135862** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/135862?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a> 👈 <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/135862?utm_source=stack-comment-view-in-graphite" target="_blank">(View in Graphite)</a>
* `main`


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