[clang] [clang] Proofread UsersManual.rst (PR #164928)

[LLVM Continuous Integration via cfe-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` at step 3 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/41872

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

```
Step 3 (clean-build-dir) failure: Delete failed. (failure) (timed out)
Step 4 (cmake-configure) failure: cmake (failure) (timed out)
command timed out: 1200 seconds without output running [b'cmake', b'-DLLVM_TARGETS_TO_BUILD=PowerPC', b'-DLLVM_INSTALL_UTILS=ON', b'-DCMAKE_CXX_STANDARD=17', b'-DLLVM_LIT_ARGS=-vj 256', b'-DFLANG_ENABLE_WERROR=ON', b'-DLLVM_ENABLE_ASSERTIONS=ON', b'-DCMAKE_C_COMPILER_LAUNCHER=ccache', b'-DCMAKE_CXX_COMPILER_LAUNCHER=ccache', b'-DLLVM_ENABLE_PROJECTS=llvm;clang;mlir;flang', b'-DLLVM_ENABLE_RUNTIMES=openmp;flang-rt', b'-DCMAKE_BUILD_TYPE=Release', b'-GNinja', b'../llvm-project/llvm'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1200.359140

```

</details>

https://github.com/llvm/llvm-project/pull/164928