Adding a tooltip above Categories postbox in Post Editor

Place this code in your functions.php theme (or child theme) file. It uses jQuery to add a new box above the Category div (<div id="categorydiv" class="postbox " >).

add_action( 'admin_footer-post.php',     'wpse_99252_add_categories_title_attribute' );
add_action( 'admin_footer-post-new.php', 'wpse_99252_add_categories_title_attribute' );

/**
 * Add a title attribute to categories box in the Post Editor.
 */
function wpse_99252_add_categories_title_attribute() { ?>
    <script type="text/javascript">

        // Setup tool tip box.
        jQuery( "#categorydiv" ).before( '<center id="categorydiv-tooltip">Which catgeory should I post in?</center>' );

        // Style it. Hide it.
        jQuery( "#categorydiv-tooltip" ).css( {
             'margin-bottom': '4px',
             visibility: 'hidden'
             // Add more styles here.
        } );

        // Display or hide tool tip box on mouse events.
        jQuery( "#categorydiv" ).mouseenter( function() {
            jQuery( "#categorydiv-tooltip" ).css( 'visibility', 'visible' );

        } ).mouseleave( function() {
            jQuery( "#categorydiv-tooltip" ).css( 'visibility', 'hidden' );
        } );

    </script>
    <?php
}