[clang] [clang][doc]: Merge entries with duplicate content. (PR #134089)

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YLChenZ wrote:

Why is that? It's strange, the first patch I just merged the same Heading, and then according to your suggestion, the second patch merged the same Content entries. Then the third patch: did not include Undocumented. I used the build steps you gave to get this:

![屏幕截图 2025-04-07 205630](https://github.com/user-attachments/assets/8f7a7455-67a3-4a74-8171-b9c8027de9b9)

And I used another way to get the documentation, running directly in the `~/llvm-project/build` directory :
```
ylchen at lambda:~/llvm-project$ cd build
ylchen at lambda:~/llvm-project/build$ ninja docs-clang-html
```

I found the target file in the `~/llvm-project/build/tools/clang/docs/html` directory, like this: 

![image](https://github.com/user-attachments/assets/52bc8c18-9793-4de7-91a7-24a81edb4cbd)



https://github.com/llvm/llvm-project/pull/134089