How to change the descriptive text on the menus admin page?

The text you are referring to is located in here: wp-admin/nav-menus.php. But we will not edit directly. We will use functions.php for the convenience of preserving the changes after an upgrade.

Use the following code in functions.php

add_filter(  'gettext',  'wps_translate_words_array'  );
add_filter(  'ngettext',  'wps_translate_words_array'  );
function wps_translate_words_array( $translated ) {

 $words = array(
                    // 'word to translate' = > 'translation'
                    'Click the arrow on the right of the item to reveal additional configuration options.' => 'The maximum top level pages is 6',
                );

 $translated = str_ireplace(  array_keys($words),  $words,  $translated );
 return $translated;
}