[clang] Revise the modules document for clarity (PR #90237)

[Chuanqi Xu via cfe-commits]

================
@@ -216,51 +198,56 @@ We explain the options in the following sections.
 How to enable standard C++ modules
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Currently, standard C++ modules are enabled automatically
-if the language standard is ``-std=c++20`` or newer.
+Standard C++ modules are enabled automatically if the language standard is
+``-std=c++20`` or newer.
 
 How to produce a BMI
 ~~~~~~~~~~~~~~~~~~~~
 
-We can generate a BMI for an importable module unit by either ``--precompile``
-or ``-fmodule-output`` flags.
+To generate a BMI for an importable module unit, use either the ``--precompile``
+or ``-fmodule-output`` command line option.
 
-The ``--precompile`` option generates the BMI as the output of the compilation and the output path
-can be specified using the ``-o`` option.
+The ``--precompile`` option generates the BMI as the output of the compilation
+and the output path can be specified using the ``-o`` option.
 
-The ``-fmodule-output`` option generates the BMI as a by-product of the compilation.
-If ``-fmodule-output=`` is specified, the BMI will be emitted the specified location. Then if
-``-fmodule-output`` and ``-c`` are specified, the BMI will be emitted in the directory of the
-output file with the name of the input file with the new extension ``.pcm``. Otherwise, the BMI
-will be emitted in the working directory with the name of the input file with the new extension
+The ``-fmodule-output`` option generates the BMI as a by-product of the
+compilation. If ``-fmodule-output=`` is specified, the BMI will be emitted to
+the specified location. If ``-fmodule-output`` and ``-c`` are specified, the
+BMI will be emitted in the directory of the output file with the name of the
+input file with the extension ``.pcm``. Otherwise, the BMI will be emitted in
+the working directory with the name of the input file with the extension
----------------
ChuanqiXu9 wrote:

e.g,

```
clang++ a.cpp -c -o result/a.o
```

then `.` is the `working directory` and `./result` is the directory of the output file.

https://github.com/llvm/llvm-project/pull/90237