Reference docs instead of issue for CodeReuse/ActiveRecord cop

What does this MR do and why?

We link to the original issue for the CodeReuse/ActiveRecord message. Whilst this is valuable for context, new contributors (like myself) who first encounter this are more likely to be looking for guidelines on what boundaries exist and how they should refactor their code.

To address this we should instead link to the docs page for reusing abstractions.

A potential downside, is that the docs may change and this would then become outdated. Though in that case hopefully someone would notice the broken link and fix it.

An alternative is I post to that page in the issue, for any new devs to find.