Augment machine learning forecast details #975

There are some details related to forecasts that should be clarified in one of the following locations:

• https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-forecast.html
• https://www.elastic.co/guide/en/machine-learning/current/ml-overview.html
• https://www.elastic.co/guide/en/machine-learning/current/ml-gs-forecasts.html

For example:

1. Clarify that forecast is meant to be used from Kibana.
2. Clarify how to know when a forecast is ready for viewing.
3. Clarify how to stop forecasts.

There is also a problem with the screenshot in https://www.elastic.co/guide/en/machine-learning/current/ml-overview.html, since it doesn't show a forecast.