You should never edit the core files of WordPress.

Instead you should only ever edit Plugin files and or Theme files (Plugins or Themes folder)

One of the easiest was to achieve this would be via jQuery;

jQuery('<div/>', {
    id: 'your-note',
    text: 'Add up to 5 tags...etc'
}).appendTo('#tagsdiv-post_tag .inner');

Save the above into a alerts.js file (or name it whatever you want). Place this file in your theme folder /theme_name/js/ for organizational purposes.

Then in your functions.php file paste;

function load_my_alerts(){
      wp_register_script( 
        'my_alerts', 
        get_template_directory_uri() . '/js/alerts.js', 
        array( 'jquery' )
    );
    wp_enqueue_script( 'my_alerts' );
}
add_action('admin_enqueue_scripts', 'load_my_alerts');

This will enqueue and load your alerts.js file only in the admin section of your site and as such it will then apply your jQuery by appending a div with an ID selector of your choice followed by your desired text.

This part ('<div/>', { will create a self-enclosed div for you, i.e. <div id="your-note">your note here...</div>

UPDATE

You might in effect try this as a template for multiple items;

jQuery(document).ready(function($){

$('<div/>', {
    id: 'xxx',
    text: 'yyy'
}).appendTo('#tagsdiv-post_tag .inner');

$('<div/>', {
    id: 'aaa',
    text: 'bbb'
}).appendTo('#some_other_element .example');

// etc...

});