Document that foreign keys may not be used by all drivers

I am using 'mysql Ver 14.12 Distrib 5.0.70, for pc-linux-gnu (x86_64) using readline 5.2'.

Trying to create a foreign key does not seem to be working. The code executes with no errors and the table is constructed but the foreign key does not exist. Replacing a valid lookup table name with garbage also does not throw an error and the table is still constructed leaving me to believe that it is simply ignoring this altogether.

From the MySQL docs (http://dev.mysql.com/doc/refman/5.0/en/ansi-diff-foreign-keys.html):
For storage engines other than InnoDB, MySQL Server parses the FOREIGN KEY syntax in CREATE TABLE statements, but does not use or store it. In the future, the implementation will be extended to store this information in the table specification file so that it may be retrieved by mysqldump and ODBC. At a later stage, foreign key constraints will be implemented for MyISAM tables as well.

My tables are set to be InnoDB by default and not MyISAM so the foreign key constraint should not be ignored.

  $schema['lookup_table'] = array(
    'description' => 'This is the table that will contain the lookup data.',
    'fields' => array(
      'id' => array(
        'description' => 'Primary key for lookup data.',
        'type' => 'varchar',
        'length' => 4,
        'not null' => TRUE,
        'default' => ''),
      'name' => array(
        'description' => 'Name of lookup item.',
        'type' => 'varchar',
        'length' => 60,
        'not null' => TRUE,
        'default' => ''),
    ),
    'primary key' => array('id'),
  );
  $schema['main_table'] = array(
    'description' => 'This is the table that will reference the lookup table.',
    'fields' => array(
      'id' => array(
        'description' => 'Primary key for main table.',
        'type' => 'varchar',
        'length' => 9,
        'not null' => TRUE,
        'default' => ''),
      'lookup_id' => array(
        'description' => 'ID of lookup item.',
        'type' => 'varchar',
        'length' => 4,
        'not null' => TRUE,
        'default' => ''),
    ),
    'foreign keys' => array(
      'lookup_table_foreign_key' => array(
        'table' => 'lookup_table',
        'columns' => array('lookup_id' => 'id'),
      ),
    ),
    'primary key' => array('id'),
  );

I've tried to reverse the field names in the columns entry as I could not tell from the documentation (http://drupal.org/node/146939) which is to come first but that did not help either.