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

[Timm Baeder via cfe-commits]

tbaederr wrote:

Does this actually show up at the right place? Or is it now the documentation for `path_empty()`? Or  even `path_iterator`?

https://github.com/llvm/llvm-project/pull/85623