etcdctl, etcdctlv3: add help message for non-valid value arg

Unix commands interprets argument value of first character '-'
as a flag. And this won't be fixed in our upstream CLI libraries.
This adds help message to show users workarounds.

Addressing https://github.com/coreos/etcd/issues/2020.

etcdctl/command/set_command.go

@@ -29,6 +29,12 @@ func NewSetCommand() cli.Command {
 		Name:      "set",
 		Usage:     "set the value of a key",
 		ArgsUsage: "<key> <value>",
+		Description: `Set sets the value of a key.
+
+   When <value> begins with '-', <value> is interpreted as a flag.
+   Insert '--' for workaround:
+
+   $ set -- <key> <value>`,
 		Flags: []cli.Flag{
 			cli.IntFlag{Name: "ttl", Value: 0, Usage: "key time-to-live"},
 			cli.StringFlag{Name: "swap-with-value", Value: "", Usage: "previous value"},