Improve help text on email field about how emails are used with this module

I think, this might be a nice novice task to solve. See #7

As of #6

We should make sure to change the description, if the submodule is active, as it uses your email-address as your username and usernames can leak: https://www.drupal.org/drupal-security-team/security-team-procedures/dis... → and this text:

The email address is not made public and will only be used if you wish to receive a new password or wish to receive certain news or notifications by email.

won't fully apply then.

I'd say the core field description is enough?

The email address is not made public. It will only be used if you need to be contacted about your account or for opted-in notifications.

I don't think this needs any further adjustment (aside from the submodule).