posix.AT.FDCWD is not the return type of std.fs.cwd(), it’s the value that the fd field of the std.fs.Dir returned from cwd() is set to on non-Windows/non-WASI platforms (see this recent thread for more info). The Declaration not found is still a bug in the docs, though.
The return type of std.fs.cwd() is std.fs.Dir.
