docs: route element must specify network address
docs: route element must specify network address

because network address is required by route, so
here we should add one avoid user misunderstand.

Signed-off-by: Chen Fan <12d549d2a7ab71d88ff3fc5139832541597eadf9@cn.fujitsu.com>

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