Cron API is not documented for alerting

[stockholmux]

We had a question on the forums with regards to setting up an alert to run at a specific time - the answer was, of course, cron.

On the API page all examples of schedule are using period/interval and none use cron/expression.

The Cron expression reference shows how to write cron patterns (useful, although not unique to this software), but it doesn't show how you use this in context of the alerting plugin.

I had to setup an alert in Kibana with a cron pattern then look at the response in the DevTools api to find the proper JSON syntax - the average user would not do this.

[aetter]

Looks to be included as part of #463