[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 `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `clang,llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/18501

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
18.787 [4984/36/1704] Building OtoolOpts.inc...
18.788 [4984/35/1705] Building WindresOpts.inc...
18.788 [4984/34/1706] Linking CXX static library lib/libMLIRPDLLODS.a
18.790 [4983/34/1707] Linking CXX static library lib/libFortranSupport.a
18.792 [4983/33/1708] Building CXX object tools/llvm-mt/CMakeFiles/llvm-mt.dir/llvm-mt.cpp.o
18.793 [4983/32/1709] Building CXX object tools/llvm-mt/CMakeFiles/llvm-mt.dir/llvm-mt-driver.cpp.o
18.794 [4982/32/1710] Creating library symlink lib/libmlir_arm_sme_abi_stubs.so
18.795 [4982/31/1711] Building BuiltinTypes.h.inc...
18.799 [4980/32/1712] Creating library symlink lib/libmlir_float16_utils.so
18.805 [4978/33/1713] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/tools/clang/include/clang/Driver/Options.inc 
cd /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build && /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/llvm-tblgen -gen-opt-parser-defs -I /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/clang/include/clang/Driver -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/clang/include -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/tools/clang/include -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/include -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/include /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/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/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
18.805 [4978/32/1714] Creating library symlink lib/libmlir_arm_runner_utils.so
18.808 [4978/31/1715] Building BitcodeStripOpts.inc...
18.846 [4978/30/1716] Linking CXX static library lib/libTableGenLspServerLib.a
18.853 [4978/29/1717] Linking CXX static library lib/libMLIRPDLLAST.a
18.858 [4978/28/1718] Linking CXX static library lib/libMLIRSparseTensorRuntime.a
18.869 [4978/27/1719] Building BuiltinTypeConstraints.h.inc...
18.872 [4978/26/1720] Building BuiltinTypeConstraints.cpp.inc...
18.874 [4978/25/1721] Building SPIRVSerialization.inc...
18.888 [4978/24/1722] Linking CXX shared library lib/libmlir_async_runtime.so.20.0git
18.928 [4978/23/1723] Building LLVMConversionEnumsFromLLVM.inc...
18.933 [4978/22/1724] Building LLVMConversionEnumsToLLVM.inc...
18.936 [4978/21/1725] Building LLVMConversions.inc...
18.947 [4978/20/1726] Building LLVMOpFromLLVMIRConversions.inc...
18.985 [4978/19/1727] Linking CXX executable bin/mlir-src-sharder
18.994 [4978/18/1728] Linking CXX shared library lib/libmlir_runner_utils.so.20.0git
19.010 [4978/17/1729] Building TestOpsSyntax.cpp.inc...
19.011 [4978/16/1730] Building TestOpsSyntax.h.inc...
19.054 [4978/15/1731] Linking CXX executable bin/llvm-mt
21.117 [4978/14/1732] Building PPCGenCallingConv.inc...
21.311 [4978/13/1733] Building PPCGenExegesis.inc...
21.322 [4978/12/1734] Building PPCGenRegisterBank.inc...
21.377 [4978/11/1735] Building PPCGenRegisterInfo.inc...
21.386 [4978/10/1736] Building PPCGenDisassemblerTables.inc...
21.412 [4978/9/1737] Building PPCGenAsmWriter.inc...
21.508 [4978/8/1738] Building PPCGenMCCodeEmitter.inc...
21.581 [4978/7/1739] Building PPCGenAsmMatcher.inc...
22.210 [4978/6/1740] Building PPCGenSubtargetInfo.inc...
22.750 [4978/5/1741] Building AArch64TargetParserDef.inc...
23.642 [4978/4/1742] Building PPCGenFastISel.inc...
24.240 [4978/3/1743] Building PPCGenGlobalISel.inc...
24.372 [4978/2/1744] Building PPCGenDAGISel.inc...
24.579 [4978/1/1745] Building PPCGenInstrInfo.inc...
ninja: build stopped: subcommand failed.

```

</details>

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