[llvm] [CaptureTracking][NFC] Clarify usage expectations in PointerMayBeCaptured comments (PR #132744)

[via llvm-commits]

Camsyn wrote:

Since I didn’t see the comment update in the latest commit yet, I’ve gone ahead and included it in this PR — hope that’s okay. 

https://github.com/llvm/llvm-project/pull/132744