The documentation can be set for each chart object.
If you use Master Items it is possible to add a description to the Dimension and Expression as well as chart.
In the General Tab of the chart enable the Show details option.
In the chart, it will add the i icon for information on the Top right side of the chart.,
If you click on the icon, a window will show the descriptions from Master items
I did it. The hover shows the Title of the Chart. I wanted to show the description upon hovering. Also I am not able to see i icon. Can you please list the exact steps or attach a sample app how to do that?