Add comment.

message.go

@@ -1,9 +1,12 @@
 package fcm
 
 type Message struct {
+	// Union field target can be only one of the following:
 	Token     string `json:"token,omitempty"`
 	Topic     string `json:"topic,omitempty"`
 	Condition string `json:"condition,omitempty"`
+	// End of list of possible types for union field target.
+
 	Data         map[string]string `json:"data,omitempty"`
 	Notification *Notification     `json:"notification,omitempty"`
 	FcmOptions   *FcmOptions       `json:"fcm_options,omitempty"`