[clang] [Clang] Clarify diagnostic notes for implcitly generated deduction guides (PR #96084)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

(A screenshot that helps to understand the changes here)
![ctad](https://github.com/llvm/llvm-project/assets/117258288/09d3d4b0-307b-4d8e-ae85-5f13734a2d07)

(compared to what we have now and what GCC gives: https://godbolt.org/z/948M7x7fE)

https://github.com/llvm/llvm-project/pull/96084