apibuild: Add index.warning() method
apibuild: Add index.warning() method

This method is used in eg. index.merge(), but is not defined
anywhere. The implementation has been copied from docBuilder.

VenkatDatta/libvirt,libvirt/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,zippy2/libvirt,olafhering/libvirt,datto/libvirt,taget/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,eskultety/libvirt,nertpinx/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,datto/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,taget/libvirt,eskultety/libvirt,zippy2/libvirt,libvirt/libvirt,libvirt/libvirt,jardasgit/libvirt,jfehlig/libvirt,taget/libvirt,jfehlig/libvirt,crobinso/libvirt,datto/libvirt,jardasgit/libvirt,crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,olafhering/libvirt,taget/libvirt,VenkatDatta/libvirt,olafhering/libvirt,datto/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,eskultety/libvirt,datto/libvirt,jardasgit/libvirt,jfehlig/libvirt,taget/libvirt,andreabolognani/libvirt,eskultety/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,olafhering/libvirt,zippy2/libvirt,eskultety/libvirt,crobinso/libvirt,fabianfreyer/libvirt,jfehlig/libvirt