Show custom message/info for authors in add/edit post page

You can do this by hooking into the edit-post.php page. There are a variety of hooks you could use, for example, if you want to display a message below the edit title field, you could hook into edit_form_after_title.

See the example code I have below with comments:

add_action(
  'edit_form_after_title', // the hook
  'wpse_228208' // the function
);

function wpse_228208() {

  global $post;

  // confirm if the post_type is 'post'
  if ($post->post_type!== 'post')
    return;

  // you can also perform a condition for post_status, for example, if you don't want to display the message if post is already published:
  if ($post->post_status!== 'publish')
    return;

   // here goes your message
   echo '<div style="">Hello world!</div>';
}