Augment machine learning forecast details

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.

首先比较 ml-forecast.html、ml-overview.html 和 ml-gs-forecasts.html 中关于 forecast 的指南。验证 Kibana 工作流、就绪状态指示、停止行为和概览截图；当相关页面清楚地解释这些要点，且截图显示 forecast 时，工作即告完成。

由索引模型根据 Issue 内容生成。