[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 `lldb-x86_64-debian` running on `lldb-x86_64-debian` while building `clang,llvm` at step 4 "build".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/162/builds/15109

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

```
Step 4 (build) failure: build (failure)
1.022 [308/3/1] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /home/worker/2.0.1/lldb-x86_64-debian/build/tools/clang/include/clang/Driver/Options.inc 
cd /home/worker/2.0.1/lldb-x86_64-debian/build && /home/worker/2.0.1/lldb-x86_64-debian/build/bin/llvm-tblgen -gen-opt-parser-defs -I /home/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/include/clang/Driver -I/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/include -I/home/worker/2.0.1/lldb-x86_64-debian/build/tools/clang/include -I/home/worker/2.0.1/lldb-x86_64-debian/build/include -I/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/include /home/worker/2.0.1/lldb-x86_64-debian/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/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
1.026 [308/2/2] Generating VCSRevision.h
2.652 [308/1/3] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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