It might be better to add a placeholder to the filter using a data-placeholder
attribute on the table header (demo)
<th data-placeholder="Search Alphanumeric">AlphaNumeric</th>
But if you truly want a tooltip to appear, then try this code (demo):
HTML
<th data-filter-title="Search Alphanumeric">AlphaNumeric</th>
Script
$('table')
.on('filterInit', function() {
var c = this.config;
c.$headers.each(function(i){
c.$filters.eq(i).attr( 'title', $(this).attr('data-filter-title') );
});
})
.tablesorter({
theme: 'blue',
widgets: ['zebra', 'filter']
});