Document default, pre-selected scopes in config field help text

Problem/Motivation

There is no documentation on project page or in the README about the pre-selected scopes. The scopes are defined per implementation, and so they are missing from the user interface help text where scopes may be added.

Proposed resolution

Add a note to the "Scopes for API call" configuration field help text including the hard-coded scopes.

And, document how a plugin might easily supplement this documentation.

Remaining tasks

• [ ] Refactor Drupal\social_auth_discord\DiscordAuthManager::getAuthorizationUrl() $scopes to be available via static method or property.
• [ ] List the scopes in the config form field help text.
• [ ] Document how an implementor module could manipulate this help text with any pertinent, additional notes (via hook or otherwise).