Don't configure HTTP/2 in http.Server.Serve(net.Listener) if the
Server's TLSConfig is set and doesn't include the "h2" NextProto
value. This avoids mutating a *tls.Config already in use if
previously passed to tls.NewListener.

Also document this. (it's come up a few times now)

Fixes #15908

Change-Id: I283eed82fdb29a791f80d801aadd9f75db244de0
Reviewed-on: https://go-review.googlesource.com/24508Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>