You can't put tooltips on the axis values. You can put annotations there, though, and they work similarly to tooltips:
var data = new google.visualization.DataTable();
data.addColumn('string', 'month');
data.addColumn({'type': 'string', 'role': 'annotation'});
data.addColumn({'type': 'string', 'role': 'annotationText'});
data.addColumn('number', 'Sold Pencils');
data.addRows([
['FEB', 'foo', 'febuary', 10],
['JAN', 'bar', 'xhi', 2310]
]);