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

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martinboehme wrote:

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

Hm, good question. This seemed the most logical place to put the documentation in the source code, but of course, once it gets processed by Doxygen, it will probably get attached to something we don't want.

I've moved the documentation down, right above the declaration of `path()`. WDYT?

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