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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I thought we decided to merge everything but the undocumented attributes? I'm still seeing `cf_returns_retained` that's distinct from `cf_returns_not_retained`, etc.

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