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/36330Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>