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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `cross-project-tests-sie-ubuntu` running on `doug-worker-1a` while building `clang,llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/181/builds/12596

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

```
Step 5 (build-unified-tree) failure: build (failure)
0.015 [299/3/1] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
0.016 [298/3/2] Generating VCSRevision.h
0.031 [295/5/3] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/build/tools/clang/include/clang/Driver/Options.inc 
cd /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/build && /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/build/bin/llvm-tblgen -gen-opt-parser-defs -I /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/clang/include/clang/Driver -I/home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/clang/include -I/home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/build/tools/clang/include -I/home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/build/include -I/home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/llvm/include /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/clang/include/clang/Driver/Options.td --write-if-changed -o tools/clang/include/clang/Driver/Options.inc -d tools/clang/include/clang/Driver/Options.inc.d
/home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
0.062 [295/4/4] Linking CXX executable bin/llvm-config
2.734 [295/3/5] Building CXX object lib/Object/CMakeFiles/LLVMObject.dir/IRSymtab.cpp.o
13.230 [295/2/6] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
13.816 [295/1/7] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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