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

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

LGTM, I'm glad to see this documentation improvement :smile:

My only comments are small nitpicks.

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