[clang] [docs] [C++20] [Modules] Ideas for transitioning to modules (PR #80687)

[David Blaikie via cfe-commits]

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@@ -610,6 +610,345 @@ the following style significantly:
 
 The key part of the tip is to reduce the duplications from the text includes.
 
+Ideas for converting 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
+interfaces for a while to not break existing users.
+Here we provide some ideas to ease the transition process for existing libraries.
+**Note that the this section is only about helping ideas instead of requirement from clang**.
----------------
dwblaikie wrote:

```suggestion
**Note that the this section is only about helpful ideas instead of requirement from clang**.
```

https://github.com/llvm/llvm-project/pull/80687