outdated docs for onion_extend_cpath()
The description of `onion_extend_cpath()` hasn't been accurate since [f2bd0e2f16944de3b3c8722fb9412ad60d0fae2c](https://gitweb.torproject.org/tor.git/commit/?id=f2bd0e2f16944de3b3c8722fb9412ad60d0fae2c) changed its arguments.
**Trac**:
**Username**: cyberpunks
issue