Update API docs

@bparees @VojtechVitek using the syntax above, how you will compose something like this:

```
{
  "expression": "[api_key][rand:[a-z]{3}]"
}
```

Like, 'api_key' expands to APIKeyGenerator which do a HTTP request and return some API_KEY and then append 5 random characters in the end. (Just thinking ;-)