[WIP] Initial documentation of nvim features

Starting some documentation so users read about Neovim from within nvim!
Suggestion:

```
Sends {event} to {channel} via |msgpack-rpc-api| and returns immediately.
If {channel} is 0, the event is broadcast to all channels. For example:
```

The implementation details regarding "msgpack" can be left out and delegated to the `|msgpack-rpc-api|` tag.