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

[Vlad Serebrennikov via cfe-commits]

================
@@ -738,22 +736,21 @@ the following style significantly:
   import M;
   ... // use declarations from module M.
 
-The key part of the tip is to reduce the duplications from the text includes.
+Reducing the duplication from textual includes is what improves compile-time
+performance.
 
-Ideas for converting to modules
--------------------------------
+Transitioning to modules
+------------------------
 
-For new libraries, we encourage them to use modules completely from day one if possible.
-This will be pretty helpful to make the whole ecosystems to get ready.
-
-For many existing libraries, it may be a breaking change to refactor themselves
-into modules completely. So that many existing libraries need to provide headers and module
+New code and libraries should use modules from day one if possible. However,
----------------
Endilll wrote:

```suggestion
New code and libraries should use modules from day 1 if possible. However,
```

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