Add explanation to configuration page

[izmeez]

The configuration page needs some text to explain what "Roles that are considered "administrators" for masquerading" means. This explanation was omitted in the Drupal 7 version of the module but existed in the Drupal 6 version. It is a way to use an extra permission to protect these roles. Here is the text as a start that can be improved on:

Only the users with masquerade as admin permission, will be able to masquerade as the users who belong to the roles selected below. User #1 is automatically considered an administrator, regardless of roles.

The last sentence is possibly unnecessary and confusing as is.