[Feature] Document tooltip events

### What problem does this feature solve?

The documentation on `tooltip.showContent` says: "It should be configurated to be false, if you only need tooltip to trigger the event or show the axisPointer without content."
However there's no tooltip-triggered event documented. `showTip` and `hideTip` are only listed as actions, although you actually can listen to these two events.

`updateAxisPointer` is another event that exists but is not documented, also previously mentioned in #18936.

I want to use these events to let users click on the chart (even in empty places) when an axis-triggered tooltip is shown and perform some action depending on the axisPointer value. I previously did this by reading the value in a custom tooltip formatter and check whether the position the user clicked on is inside the grid area.

### What does the proposed API look like?

Just document the events, no API changes needed.