MVC - explain invokeInterfaceRegistration in ApiMutableServiceControllerBase

[AdSchellevis]

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• [x] I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• [x] I have searched the existing issues and I am convinced that mine is new.

Is your feature request related to a problem? Please describe.

documentation for https://github.com/opnsense/core/issues/5768

[AdSchellevis]

https://github.com/opnsense/docs/commit/94347898455523e320fa1d2e98626290303194fb