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Here are two examples using the pre-upload-ui and post-upload-ui hooks:

add_action( 'pre-upload-ui', function() {  
    ?><h4>
         <?php esc_html_e( 'Please Remember This And That! (pre-upload-ui)', 'wpse' );?>
      </h4>
    <?php 
});

add_action( 'post-upload-ui', function() {  
    ?><h4>
         <?php esc_html_e( 'Please Remember This And That! (post-upload-ui)', 'wpse' );?>
      </h4>
    <?php 
});

You might want to adjust this further to follow the Admin UI style.

This will show up as:

and