[clang] [clang] Document the type_visibility attribute (PR #79157)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for the improved documentation! In general, heading in the right direction. There might be some more details we'd like to add, but if you don't know those details, that's totally fine (this at least moves the needle in the right direction).

https://github.com/llvm/llvm-project/pull/79157