Categorize RPC help overview

Didn't make it into 0.9.3, but the overview is much more well-arranged nevertheless:

![jv3clmh](https://cloud.githubusercontent.com/assets/5836089/4617728/72ac4592-5302-11e4-9002-0555c734f3ae.png)

I may revert 6a6231d, so the change is purely visual.