Make clear negotiation can happen via NPN or ALPN, similar to
http.Transport.TLSNextProto and x/net/http2.NextProtoTLS.

Change-Id: Ied00b842bc04e11159d6d2107beda921cefbc6ca
Reviewed-on: https://go-review.googlesource.com/23108Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>