1. When running locally, found that these **TODO** json-rpc have been
implemented. Updating these outdated information can make the document
clearer.
```
curl -X POST -H "Content-Type: application/json" --data '{"jsonrpc":"2.0","method":"eth_newBlockFilter","params":[],"id":1}' http://loca
lhost:8545
{"jsonrpc":"2.0","id":1,"result":"0x02000000000000009c49b60f431cabc7"}
```
2. [but it's easy to accomplish] Points to an obsolete message
(9b8cdc0f22/eth/downloader/downloader.go (L673))
