[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 `flang-aarch64-release` running on `linaro-flang-aarch64-release` while building `clang,llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/172/builds/8899

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
13.969 [3681/21/3116] Building AttrPCHWrite.inc...
13.969 [3681/20/3117] Building Nodes.inc...
13.969 [3681/19/3118] Building NodeClasses.inc...
13.969 [3681/18/3119] Building Checkers.inc...
13.969 [3681/17/3120] Building arm_fp16.h...
13.970 [3681/16/3121] Building arm_bf16.h...
13.970 [3681/15/3122] Building arm_cde.h...
13.972 [3681/14/3123] Building arm_vector_types.h...
13.973 [3681/13/3124] Building AttrParsedAttrImpl.inc...
13.974 [3681/12/3125] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /home/tcwg-buildbot/worker/flang-aarch64-release/build/tools/clang/include/clang/Driver/Options.inc 
cd /home/tcwg-buildbot/worker/flang-aarch64-release/build && /home/tcwg-buildbot/worker/flang-aarch64-release/build/bin/llvm-tblgen -gen-opt-parser-defs -I /home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/clang/include/clang/Driver -I/home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/clang/include -I/home/tcwg-buildbot/worker/flang-aarch64-release/build/tools/clang/include -I/home/tcwg-buildbot/worker/flang-aarch64-release/build/include -I/home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/llvm/include /home/tcwg-buildbot/worker/flang-aarch64-release/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/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
13.977 [3681/11/3126] Building arm_sme.h...
14.005 [3681/10/3127] Building arm_mve.h...
14.044 [3681/9/3128] Building arm_sve.h...
14.073 [3681/8/3129] Building riscv_vector_builtins.inc...
14.078 [3681/7/3130] Building riscv_vector_builtin_sema.inc...
14.110 [3681/6/3131] Building arm_neon.h...
14.137 [3681/5/3132] Building riscv_vector_builtin_cg.inc...
14.282 [3681/4/3133] Building CXX object lib/Object/CMakeFiles/LLVMObject.dir/IRSymtab.cpp.o
17.545 [3681/3/3134] Building CXX object lib/MC/MCParser/CMakeFiles/LLVMMCParser.dir/AsmParser.cpp.o
23.246 [3681/2/3135] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
27.555 [3681/1/3136] 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