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

[Erich Keane via cfe-commits]

================
@@ -925,45 +923,41 @@ In that case, you need to convert your source files (.cpp files) to module imple
   // Following off should be unchanged.
   ...
 
-The module implementation unit will import the primary module implicitly.
-We don't include any headers in the module implementation units
-here since we want to avoid duplicated declarations between translation units.
-This is the reason why we add non-exported using declarations from the third
-party libraries in the primary module interface unit.
+The module implementation unit will import the primary module implicitly. Do
+not include any headers in the module implementation units because that avoids
+duplicated declarations between translation units. This is why non-exported
+using declarations are added from third-party libraries in the primary module
----------------
erichkeane wrote:

```suggestion
using declarations should be added from third-party libraries in the primary module
```

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