[clang] [llvm] [Clang] Cleanup docs and comments relating to -fextend-variable-liveness (PR #124767)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Apologies, I didn't expect this PR to trigger a failure and didn't catch the triggered failures; thank you for catching this so quickly!

https://github.com/llvm/llvm-project/pull/124767