[clang] [docs][coroutines] Revamp "Debugging C++ coroutines" (PR #142651)

[Jan Finis via cfe-commits]

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@@ -8,470 +8,966 @@ Debugging C++ Coroutines
 Introduction
 ============
 
-For performance and other architectural reasons, the C++ Coroutines feature in
-the Clang compiler is implemented in two parts of the compiler.  Semantic
-analysis is performed in Clang, and Coroutine construction and optimization
-takes place in the LLVM middle-end.
+Coroutines in C++ were introduced in C++20, and their user experience for
----------------
JFinis wrote:

```suggestion
Coroutines in C++ were introduced in C++20, and the user experience for
```

https://github.com/llvm/llvm-project/pull/142651