fixed note about post redirect get

Fixed guide example by adding note about redirect / refresh . See this [comments](https://github.com/yiisoft/yii2/commit/0e18fbf89331305d5caed7ae5aafe176154bc1d3#commitcomment-6620020) for more info .
"Note" fits there better.

prompt confirmation dialog → show confirmation prompt

refresh and redirect should be linked to API docs