Development guideline Docs: Not everything is a service object

What does this MR do?

Not everything is a service object

Clarify what is a service object, and alternatives if the object is not a service object

Why

The short answer is that we have a convention. The longer answer is that we do not want service classes to be a generic classes that can be anything, it needs to share a common set of traits (like passing the current_user, and having params).

Classes that do not conform to this convention probably benefits from moving away from being a service class, and not being restricted to execute interface un-necessarily.

See also https://gitlab.slack.com/archives/CJHPLRTRD/p1632278528082800 (internal), and also !66735 (comment 685940799)

/cc @gitlab-org/maintainers/rails-backend /cc @splattael @minac

Related issues

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