[clang] [Doc] Update documentation for no-transitive-change (PR #96453)

[Ilya Biryukov via cfe-commits]

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@@ -229,6 +229,10 @@ C++20 Feature Support
   will now work.
   (#GH62925).
 
+- Clang refactored the BMI format to make it possible to support no transitive changes
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ilya-biryukov wrote:

Maybe explicitly mention the user-facing effect of what "no transitive change" means and mention that we want feedback on this?

Something like:
- Clang implemented improvements to BMI of C++20 Modules that should reduce the number of rebuilds during incremental recompilation. We are seeking feedback from Build System authors and other interested users, especially when you feel Clang changes the BMI and missses an opportunity to avoid recompilations or causes correctness issues. See `StandardCPlusPlusModules <StandardCPlusPlusModules.html>`_ for more details. 

Also, maybe put this at the top to increase chances of people noticing this? (if we do want people filing bugs about it, it'd help to increase the chances that they do).

https://github.com/llvm/llvm-project/pull/96453