'updateAxisPointer' event is missing in the doc

`updateAxisPointer` is a very useful event to listen where item is hovered by mouse.

`echarts_instance.on('updateAxisPointer', e => console.log(e))` works but never be mentioned in the doc.

please add it

thaks 🍵

> event object
```json
{
"type": "updateaxispointer",
"seriesIndex": 0,
"dataIndexInside": 37,
"dataIndex": 336,
"axesInfo": [
{
"axisDim": "x",
"axisIndex": 0,
"value": 336
},
{
"axisDim": "y",
"axisIndex": 0,
"value": 5188.092254685122
},
{
"axisDim": "x",
"axisIndex": 1,
"value": 336
}
]
}
```