Documentation improvement

I had difficulties to make use of this module in order to get a form to edit my module's variables.
What made it tricky was the variable_group_info:

This works :

function mymodule_variable_group_info() {
  $groups['mymodule'] = array(  
    'title' => t('Group title'),
    'description' => t('Group description.'),
    'access' => 'some permission',
    'path' => array(' the page path matching a menu item'),
  );
  return $groups;
}

This does not work :

function mymodule_variable_group_info() {
  $groups['mymodule_group1'] = array(  
    'title' => t('Group title'),
    'description' => t('Group description.'),
    'access' => 'some permission',
    'path' => array(' the page path matching a menu item'),
  );
  return $groups;
}

In short, you can only have one group, and its name is the module name.

Hope this helps