The documentation seems to be correct. I can't extern( C++, class ) or extern( C++, struct ) on an object, even in DMD 2.071.2-beta3.
`extern( C++, class/struct )` is supported by DMD master and LDC 1.1.0-beta*. Afaik, there is no versioning of the spec, and so the spec was already updated to prevent the spec going out-of-sync with the implementation.