[clang] [clang][doc]: Merge the contents of identical entries. (PR #134089)

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YLChenZ wrote:

> Oh yeah, that is not a good outcome. :-) I think Undocumented is a special case. For this patch, I would not merge if the category is Undocumented.

@AaronBallman Oh, I forgot the other merged entry that is slightly smaller compared to `Undocumented` like this:

![屏幕截图 2025-04-04 200659](https://github.com/user-attachments/assets/f04e1099-966d-4158-9ddb-1b130a795146)


![屏幕截图 2025-04-04 195903](https://github.com/user-attachments/assets/b0f155a9-233f-4b98-b20c-8f8343516197)

Similarly, it doesn't look very pretty.


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