docs, bring back `ActiveRecord::Core` methods in the API. [ci skip]

This `# :nodoc:` had the effect of hiding every method that follows.
This meant that the API page for `ActiveRecord::Core` only contained
`configurations` and none of the following methods.

Furthermore this `# :nodoc:` had no effect on `maintain_test_schema`.
Those `mattr_accessor` inside the `included` block are not picked up
by rdoc.

/cc @zzak