[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 `clangd-ubuntu-tsan` running on `clangd-ubuntu-clang` while building `clang,llvm` at step 5 "build-clangd-clangd-index-server-clangd-indexer".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/134/builds/12547

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

```
Step 5 (build-clangd-clangd-index-server-clangd-indexer) failure: build (failure)
...
78.441 [1852/18/2874] Building CXX object lib/TextAPI/CMakeFiles/LLVMTextAPI.dir/Symbol.cpp.o
78.452 [1851/18/2875] Building CXX object lib/TextAPI/CMakeFiles/LLVMTextAPI.dir/SymbolSet.cpp.o
78.459 [1850/18/2876] Building CXX object lib/TextAPI/CMakeFiles/LLVMTextAPI.dir/Target.cpp.o
78.462 [1849/18/2877] Building CXX object lib/TextAPI/CMakeFiles/LLVMTextAPI.dir/TextAPIError.cpp.o
78.469 [1848/18/2878] Building CXX object lib/TextAPI/CMakeFiles/LLVMTextAPI.dir/TextStubCommon.cpp.o
78.470 [1848/17/2879] Building CXX object lib/TextAPI/CMakeFiles/LLVMTextAPI.dir/TextStub.cpp.o
78.474 [1848/16/2880] Building CXX object lib/TextAPI/CMakeFiles/LLVMTextAPI.dir/Utils.cpp.o
78.482 [1848/15/2881] Building CXX object lib/WindowsDriver/CMakeFiles/LLVMWindowsDriver.dir/MSVCPaths.cpp.o
78.504 [1848/14/2882] Linking CXX static library lib/libLLVMTargetParser.a
78.514 [1846/15/2883] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build/tools/clang/include/clang/Driver/Options.inc 
cd /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build && /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build/NATIVE/bin/llvm-tblgen -gen-opt-parser-defs -I /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/clang/include/clang/Driver -I/vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/clang/include -I/vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build/tools/clang/include -I/vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build/include -I/vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/llvm/include /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/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
/vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
78.563 [1846/14/2884] Linking CXX static library lib/libLLVMWindowsDriver.a
78.570 [1846/13/2885] Linking CXX static library lib/libLLVMBinaryFormat.a
78.577 [1846/12/2886] Building RISCVGenInstrInfo.inc...
78.856 [1846/11/2887] Building X86GenRegisterInfo.inc...
79.048 [1846/10/2888] Building X86GenFastISel.inc...
79.055 [1846/9/2889] Building X86GenRegisterBank.inc...
79.129 [1846/8/2890] Building AMDGPUGenRegisterInfo.inc...
79.383 [1846/7/2891] Building X86GenGlobalISel.inc...
79.500 [1846/6/2892] Building X86GenMnemonicTables.inc...
80.272 [1846/5/2893] Building RISCVGenGlobalISel.inc...
80.503 [1846/4/2894] Building X86GenDAGISel.inc...
80.980 [1846/3/2895] Building RISCVGenDAGISel.inc...
82.724 [1846/2/2896] Building X86GenInstrInfo.inc...
82.779 [1846/1/2897] Building X86GenSubtargetInfo.inc...
ninja: build stopped: subcommand failed.

```

</details>

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