[clang][NFC] Add documentation for `CastExpr::path()`. (#85623)

This didn't have any documentation, so I had to do some experimenting in
godbolt when I used this in
https://github.com/llvm/llvm-project/pull/84138, and my reviewer later
also had some
[questions](https://github.com/llvm/llvm-project/pull/84138#discussion_r1524855434)
about this, so I figured it would be worth adding documentation.