Tooltip customization is available in Winter '19 release. As per the documents:
This attribute for the URL type column property is new. tooltip — Displays a tooltip for a URL in the datatable column. Use the typeAttributes property to pass the tooltip for the URL.
Eg: For assigning a particular field name to a tooltip
tooltip: { fieldName: attributeFieldName }
tooltip: {
fieldName: attributeFieldName
}
Or for assigning a normal text to a tooltip
tooltip:'Normal text'