In the "Installing Oracle Solaris 11 Systems" guide, there's a reference to menu.lst for dual booting. That's only partially accurate, as Debian/Ubuntu-type systems are using GRUB 2.0 and therefore haven't been using menu.lst for nearly a couple of years now.
Users of such systems can consult the Linux distro documentation for instructions on how to dual-boot with legacy GRUB operating systems, or they can use chainloading from the Solaris GRUB to access the GRUB 2 loader.
You raise a valid use-case that, although isn't well covered in the current set of product documentation you've looked at, is still nevertheless important. Thank you. We are investigating ways of making this task more approachable and would expect to do so using the most flexible and appropriate of avenues, whether before (or at the release of) a Solaris 11 update. As of the time of this writing, the technical recommendation from Alta is the one that will get you there.