docs: document migrate-getmaxdowntime support
docs: document migrate-getmaxdowntime support

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