cmd/lncli: add description for payinvoice command

cmd/lncli/cmd_payments.go

@@ -806,6 +806,9 @@ var payInvoiceCommand = cli.Command{
 	Name:     "payinvoice",
 	Category: "Payments",
 	Usage:    "Pay an invoice over lightning.",
+	Description: `
+	This command is a shortcut for 'sendpayment --pay_req='.
+	`,
 	ArgsUsage: "pay_req",
 	Flags: append(paymentFlags(),
 		cli.Int64Flag{