[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 `llvm-clang-x86_64-win-fast` running on `as-builder-3` while building `clang,llvm` at step 6 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/2/builds/15908

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

```
Step 6 (build-unified-tree) failure: build (failure)
...
[929/4138] Building Opts.inc...
[930/4138] Building Opts.inc...
[931/4138] Building Opts.inc...
[932/4138] Building ARMGenGlobalISel.inc...
[933/4138] Building InstallNameToolOpts.inc...
[934/4138] Building OtoolOpts.inc...
[935/4138] Building CXX object tools\llvm-mt\CMakeFiles\llvm-mt.dir\llvm-mt.cpp.obj
[936/4138] Building CXX object tools\llvm-mt\CMakeFiles\llvm-mt.dir\llvm-mt-driver.cpp.obj
[937/4138] Building RC object tools\llvm-mt\CMakeFiles\llvm-mt.dir\__\__\resources\windows_version_resource.rc.res
[938/4138] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc C:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/build/tools/clang/include/clang/Driver/Options.inc 
cmd.exe /C "cd /D C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build && C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build\bin\llvm-tblgen.exe -gen-opt-parser-defs -I C:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/llvm-project/clang/include/clang/Driver -IC:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/llvm-project/clang/include -IC:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/build/tools/clang/include -IC:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/build/include -IC:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/llvm-project/llvm/include --long-string-literals=0 C:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/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"
C:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
[939/4138] Building WindresOpts.inc...
[940/4138] Linking CXX executable bin\llvm-opt-report.exe
[941/4138] Building BitcodeStripOpts.inc...
[942/4138] Building ARMGenDAGISel.inc...
[943/4138] Linking CXX executable bin\llvm-remarkutil.exe
[944/4138] Building ARMGenSubtargetInfo.inc...
[945/4138] Linking CXX shared library bin\Remarks.dll
[946/4138] Linking CXX executable bin\llvm-mt.exe
[947/4138] Building ARMGenInstrInfo.inc...
[948/4138] Building RISCVTargetParserDef.inc...
ninja: build stopped: subcommand failed.

```

</details>

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