Extend the Graphql/Descriptions cop to cover enum value descriptions

What does this MR do?

The existing Graphql/Descriptions cop was previously not enforcing the GraphQL description style guide for enum values. This MR changes the cop to cover GraphQL enums also.

Existing offenses have been added to .rubocop_manual_todo.yml with a follow-up issue to fix them #322903 (closed).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done