Embind: Clarify need for emscripten::internal::raw_destructor specializations
...which are needed at compile time (the emscripten::class_<T> ctor always
records the address of emscripten::internal::raw_destructor<T>, even in cases
where it never calls it; and raw_destructor internally calls delete, but the
dtor of the UNO interface types is protected) but not at runtime (as those UNO
interface types are only accessed through css::uno::Reference smart pointers)
Change-Id: I09e4f258f8dfc0fc53c0fe7210c7f709d86be176
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166304
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>