[clang][docs] Further improve the docs of the ownership_returns attribute (#191197)
This addresses:
https://github.com/llvm/llvm-project/pull/191005#discussion_r3058171126
https://github.com/llvm/llvm-project/pull/191005#discussion_r3058173173
I've reviewed the uses of "argument" and "parameter" and tried to
consolidate them to the best of my abilities.
This patch fixes up #191005