doc: sack: warning about multiple Sack usage
doc: sack: warning about multiple Sack usage

hughsie/libhif,rpm-software-management/libdnf,hughsie/libhif,cgwalters/libhif,edynox/libdnf,edynox/libdnf,Conan-Kudo/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,edynox/libdnf,hughsie/libhif,cgwalters/libhif,rpm-software-management/libhif,Conan-Kudo/libhif,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libdnf,Conan-Kudo/libhif,rpm-software-management/libhif,cgwalters/libhif,rpm-software-management/libhif,cgwalters/libhif,Conan-Kudo/libhif,edynox/libdnf