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

[Chuanqi Xu via cfe-commits]

================
@@ -312,75 +300,76 @@ So all of the following name is not valid by default:
     __test
     // and so on ...
 
-If you still want to use the reserved module names for any reason, use
-``-Wno-reserved-module-identifier`` to suppress the warning.
+Using a reserved module name is strongly discouraged, but
+``-Wno-reserved-module-identifier`` can be used to suppress the warning.
 
-How to specify the dependent BMIs
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Specifying dependent BMIs
+~~~~~~~~~~~~~~~~~~~~~~~~~
 
-There are 3 methods to specify the dependent BMIs:
+There are 3 ways to specify a dependent BMI:
----------------
ChuanqiXu9 wrote:

If we have the following code:

```
// foo.cc
import a;
...
```

then we need a BMI of module `a` to compile `foo.cc`. Here the BMI of module `a` is the dependent BMI for `foo.cc`. I feel this clear. But if we don't think so, we can add a definition for that.

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