Add detailed long description for etcdctl txn.

Include an example stdin usage and a link for full docs.

Signed-off-by: James Blair <mail@jamesblair.net>

etcdctl/ctlv3/command/txn_command.go

@@ -36,7 +36,30 @@ func NewTxnCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "txn [options]",
 		Short: "Txn processes all the requests in one transaction",
-		Run:   txnCommandFunc,
+		Long: `Txn reads multiple etcd requests from standard input and applies them as a single atomic transaction.
+
+A transaction consists of three components:
+1) a list of conditions,
+2) a list of requests to apply if all the conditions are true,
+3) a list of requests to apply if any condition is false.
+
+Example interactive stdin usage:
+
+---
+etcdctl txn -i
+# compares:
+mod("key1") > "0"
+
+# success requests (get, put, delete):
+put key1 "overwrote-key1"
+
+# failure requests (get, put, delete):
+put key1 "created-key1"
+put key2 "some extra key"
+---
+
+Refer to https://github.com/etcd-io/etcd/blob/main/etcdctl/README.md#txn-options.`,
+		Run: txnCommandFunc,
 	}
 	cmd.Flags().BoolVarP(&txnInteractive, "interactive", "i", false, "Input transaction in interactive mode")
 	return cmd