Return code from save_post action?

I’d like to understand what value I should return from a WordPress save_post action function.

This example from the save_post documentation both explicitly and implicitly returns without a value:

function my_project_updated_send_email( $post_id ) {

    // If this is just a revision, don't send the email.
    if ( wp_is_post_revision( $post_id ) )
        return;

    $post_title = get_the_title( $post_id );
    $post_url = get_permalink( $post_id );
    $subject = 'A post has been updated';

    $message = "A post has been updated on your website:nn";
    $message .= $post_title . ": " . $post_url;

    // Send email to admin.
    wp_mail( 'admin@example.com', $subject, $message );
}
add_action( 'save_post', 'my_project_updated_send_email' );

However, this example from the add_meta_box documentation very specifically returns the $post_id that was passed in if it returns early, but returns nothing if the function executes to the end.

/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
function myplugin_save_postdata( $post_id ) {

  /*
   * We need to verify this came from the our screen and with proper authorization,
   * because save_post can be triggered at other times.
   */

  // Check if our nonce is set.
  if ( ! isset( $_POST['myplugin_inner_custom_box_nonce'] ) )
    return $post_id;

  $nonce = $_POST['myplugin_inner_custom_box_nonce'];

  // Verify that the nonce is valid.
  if ( ! wp_verify_nonce( $nonce, 'myplugin_inner_custom_box' ) )
      return $post_id;

  // If this is an autosave, our form has not been submitted, so we don't want to do anything.
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
      return $post_id;

  // Check the user's permissions.
  if ( 'page' == $_POST['post_type'] ) {

    if ( ! current_user_can( 'edit_page', $post_id ) )
        return $post_id;

  } else {

    if ( ! current_user_can( 'edit_post', $post_id ) )
        return $post_id;
  }

  /* OK, its safe for us to save the data now. */

  // Sanitize user input.
  $mydata = sanitize_text_field( $_POST['myplugin_new_field'] );

  // Update the meta field in the database.
  update_post_meta( $post_id, '_my_meta_value_key', $mydata );
}
add_action( 'save_post', 'myplugin_save_postdata' );

There’s no indication on that page of why it’s returning $post_id if it returns early.

There doesn’t seem to be any actual documentation on the save_post Codex page one way or the other. I’ve seen conflicting advice around the web, too.

The example of creating an Action function in the Plugin API documentation looks like it’s also a save_post action, and returns the $post_id passed in on success, though it doesn’t say why:

function email_friends($post_ID)  {
    $friends = 'bob@example.org,susie@example.org';
    mail($friends, "sally's blog updated", 
      'I just put something on my blog: http://blog.example.com');
    return $post_ID;
}

Can anyone tell me what I, if anything, I should be returning from my save_post actions, and why, preferably with some kind of official reference?