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

[Vlad Serebrennikov via cfe-commits]

================
@@ -776,9 +773,9 @@ export-using style
     using decl_n;
   }
 
-As the example shows, you need to include all the headers containing declarations needs
-to be exported and `using` such declarations in an `export` block. Then, basically,
-we're done.
+This example shows how to include all the headers containing declarations which
+need to be exported and uses `using` declarations in an `export` block to
+produce the module interface. Then, basically, we're done.
----------------
Endilll wrote:

```suggestion
produce the module interface.
```

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