Clarify the scope of the setting in ApprovalState model spec

What does this MR do?

Follow-up to !25385 (merged).

We had multiple context disabling or enabling various self-approval rules. Within each we had another set of context for the same approval-rule but on on instance level.

context 'when self approval is enabled' do
  context 'when self approval is enabled on instance level' do
    ...
  end

  context 'when self approval is disabled on instance level' do
    ..
  end
end

This confused me at first until I realized the outer context is project level setting.

Screenshots

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team