wp_commentdata: error in documentation?

The Codex is written by humans, so it could be wrong. But what you are missing is the column »Null« beforehand, as you can see it is empty or yes. To determine what that does mean we are taking a look into the source. This you should do first if you are in doubt about the correctness of the documentation. We are looking into the schema.php file and see:

CREATE TABLE $wpdb->commentmeta (
  meta_id bigint(20) unsigned NOT NULL auto_increment,
  comment_id bigint(20) unsigned NOT NULL default '0',
  meta_key varchar(255) default NULL,
  meta_value longtext,
  PRIMARY KEY (meta_id),
  KEY comment_id (comment_id),
  KEY meta_key (meta_key)
) $charset_collate;

Which means if the column value for »Null« isn’t »Yes« it’s not NULL but NOT NULL.