net/rpc/jsonrpc: Update package doc with info about JSON-RPC 2.0

Currently the net/rpc/jsonrpc package only implements JSON-RPC version
1.0. This change updates the package's documentation with link to find
packages for JSON-RPC 2.0.

Fixes #10929

Change-Id: I3b6f1d17738a1759d7b62ab7b3ecef5b248d30ca
Reviewed-on: https://go-review.googlesource.com/36330
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/rpc/jsonrpc/client.go

@@ -2,8 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package jsonrpc implements a JSON-RPC ClientCodec and ServerCodec
+// Package jsonrpc implements a JSON-RPC 1.0 ClientCodec and ServerCodec
 // for the rpc package.
+// For JSON-RPC 2.0 support, see https://godoc.org/?q=json-rpc+2.0
 package jsonrpc
 
 import (