Update API docs

Yes, for sure. I'll work on that.

@bparees I think we didn't finish talking about the final form of  `parameters` fields (thinking of multiple generators used). Do you think the below would make better sense that the current structure? //cc @mfojtik @smarterclayton

```
{
"name": "DB_PASSWORD",
"generate": "expression",    /* or "generator" instead of "generate" */
"value": "[a-zA-Z0-9]{8}"
}
```
With the above, the json validation would be much simpler and also the errors returned would be easy to understand (an "expression" generator is not supported). Currently, we just silently ignore any extra "generators", as we don't know what fields to check.

Also, I guess, we can also remove the `"type"` field completely, as we always generate "string", right?