[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 `premerge-monolithic-linux` running on `premerge-linux-1` while building `clang,llvm` at step 6 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/21224

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

```
Step 6 (build-unified-tree) failure: build (failure)
0.008 [3601/6/1] Performing build step for 'bolt_rt'
ninja: no work to do.
0.012 [3600/6/2] No install step for 'bolt_rt'
0.014 [3599/6/3] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
0.017 [3598/6/4] Generating VCSRevision.h
0.026 [3595/8/5] Generating VCSVersion.inc
0.027 [3594/8/6] Completed 'bolt_rt'
0.027 [3594/7/7] Generating VCSVersion.inc
0.031 [3593/7/8] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /build/buildbot/premerge-monolithic-linux/build/tools/clang/include/clang/Driver/Options.inc 
cd /build/buildbot/premerge-monolithic-linux/build && /build/buildbot/premerge-monolithic-linux/build/bin/llvm-tblgen -gen-opt-parser-defs -I /build/buildbot/premerge-monolithic-linux/llvm-project/clang/include/clang/Driver -I/build/buildbot/premerge-monolithic-linux/llvm-project/clang/include -I/build/buildbot/premerge-monolithic-linux/build/tools/clang/include -I/build/buildbot/premerge-monolithic-linux/build/include -I/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/include /build/buildbot/premerge-monolithic-linux/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
/build/buildbot/premerge-monolithic-linux/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
0.088 [3593/6/9] Linking CXX executable bin/llvm-config
0.644 [3593/5/10] Building CXX object tools/flang/lib/Common/CMakeFiles/FortranCommon.dir/Version.cpp.o
1.313 [3593/4/11] Building CXX object tools/bolt/lib/Utils/CMakeFiles/LLVMBOLTUtils.dir/CommandLineOpts.cpp.o
2.710 [3593/3/12] Building CXX object lib/Object/CMakeFiles/LLVMObject.dir/IRSymtab.cpp.o
11.201 [3593/2/13] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
12.631 [3593/1/14] 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