op-node: add description to l2 genesis command

op-node/cmd/genesis/cmd.go

@@ -91,6 +91,9 @@ var Subcommands = cli.Commands{
 	{
 		Name:  "l2",
 		Usage: "Generates an L2 genesis file and rollup config suitable for a deployed network",
+		Description: "Generating the L2 genesis depends on knowledge of L1 contract addresses for the bridge to be secure. " +
+			"A deploy config and either a deployment directory or an L1 deployments file are used to create the L2 genesis. " +
+			"The deploy directory and L1 deployments file are generated by the L1 contract deployments.",
 		Flags: []cli.Flag{
 			&cli.StringFlag{
 				Name:  "l1-rpc",