net/smtp: match actual behavior for Auth.Start

Looking at history, it appears to never have worked as documented.

Fixes #48759

Change-Id: I066307c28e3ed1875c1c4049bade62e2818dd400
Reviewed-on: https://go-review.googlesource.com/c/go/+/383998
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cherry Mui <cherryyz@google.com>

src/net/smtp/auth.go

@@ -16,8 +16,7 @@ type Auth interface {
 	// Start begins an authentication with a server.
 	// It returns the name of the authentication protocol
 	// and optionally data to include in the initial AUTH message
-	// sent to the server. It can return proto == "" to indicate
-	// that the authentication should be skipped.
+	// sent to the server.
 	// If it returns a non-nil error, the SMTP client aborts
 	// the authentication attempt and closes the connection.
 	Start(server *ServerInfo) (proto string, toServer []byte, err error)