net/http: fix documentation on Server.TLSNextProto controlling http2

Server.TLSNextProto being nil is necessary but not sufficient but
http2 being automatically enabled.

Fixes #16588

Change-Id: I5b18690582f9b12ef05b58235e1eaa52483be285
Reviewed-on: https://go-review.googlesource.com/33090
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: bradfitz

src/net/http/server.go

@@ -2308,7 +2308,8 @@ type Server struct {
 	// handle HTTP requests and will initialize the Request's TLS
 	// and RemoteAddr if not already set. The connection is
 	// automatically closed when the function returns.
-	// If TLSNextProto is nil, HTTP/2 support is enabled automatically.
+	// If TLSNextProto is not nil, HTTP/2 support is not enabled
+	// automatically.
 	TLSNextProto map[string]func(*Server, *tls.Conn, Handler)
 
 	// ConnState specifies an optional callback function that is