Use Split node and Join node to manipulate your data array into string separated with newline character before sending it to be payload of Notification. Notification however does not show multiple lines. It does it magic to join lines into one and makes itself wider.
[{"id":"8c90b3f2.5949","type":"split","z":"18c83e13.bcdd42","name":"","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":560,"y":940,"wires":[["c296e15f.a2cb1"]]},{"id":"c296e15f.a2cb1","type":"join","z":"18c83e13.bcdd42","name":"","mode":"custom","build":"string","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":false,"timeout":"","count":"","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":700,"y":940,"wires":[["19900b90.8eef94"]]},{"id":"168d41d8.894a6e","type":"inject","z":"18c83e13.bcdd42","name":"","topic":"","payload":"[123,456,789]","payloadType":"json","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":390,"y":940,"wires":[["8c90b3f2.5949"]]},{"id":"19900b90.8eef94","type":"debug","z":"18c83e13.bcdd42","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":870,"y":940,"wires":[]}]