[clang] [clang][docs] Improve documentation of [[ownership_returns]] attribute (PR #191005)

[Balázs Benics via cfe-commits]

steakhal wrote:

@AaronBallman I'm going to be honest, I didn't expect your review.
My intention was to give you a heads up that what I see as next steps in this direction.

However, I figured that those discrepancies worth fixing. See them in #191197
Leave your remarks over there if you have any.

https://github.com/llvm/llvm-project/pull/191005