Improving the RPC help

Hello.
I have seen that both the hsw-cli and hsd-cli rpc interfaces are not documented:

```
$ ./hsd-cli rpc help
Select a command.
```

And fails badly when called with no arguments:

```
$ ./hsw-cli rpc
AssertionError [ERR_ASSERTION]: Assertion failed.
```

I would love to contribute, and I thought that addressing those issues could be a good start.
However, I see there's the API unification issue (#543), and could be the reason why no one looked at this behaviours yet.

What's your takes?