Document token.Wait behaviour during reconnect

When a connection is lost to the broker and later resumed the token
returned to the user will be considered completed and a new token will
be created instead. The result of this is that there is no way for a
user of this library to know whether a message has been delivered in the
presence of connection issues.

README.md

@@ -129,6 +129,7 @@ may lead to messages being silently dropped.
 * `persistence` - Defaults to false (messages will not survive a broker restart)
 * `max_keepalive` - defaults to 65535 and, from version 2.0.12, `SetKeepAlive(0)` will result in a rejected connection 
 by default.
+* `token.Wait()` - Will return successfully before a message has been delivered if the client needs to reconnect to the broker before the message has been delivered.
 
 Reporting bugs
 --------------

token.go

@@ -37,12 +37,18 @@ type PacketAndToken struct {
 type Token interface {
 	// Wait will wait indefinitely for the Token to complete, ie the Publish
 	// to be sent and confirmed receipt from the broker.
+	//
+	// Note that, for backward compatibility, if the client reconnects to the
+	// broker Wait() will return before the message is delivered
 	Wait() bool
 
 	// WaitTimeout takes a time.Duration to wait for the flow associated with the
 	// Token to complete, returns true if it returned before the timeout or
 	// returns false if the timeout occurred. In the case of a timeout the Token
 	// does not have an error set in case the caller wishes to wait again.
+	//
+	// Note that, for backward compatibility, if the client reconnects to the
+	// broker Wait() will return before the message is delivered
 	WaitTimeout(time.Duration) bool
 
 	// Done returns a channel that is closed when the flow associated
@@ -51,6 +57,9 @@ type Token interface {
 	//
 	// Done is provided for use in select statements. Simple use cases may
 	// use Wait or WaitTimeout.
+	//
+	// Note that, for backward compatibility, if the client reconnects to the
+	// broker Wait() will return before the message is delivered
 	Done() <-chan struct{}
 
 	Error() error