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

activerecord/lib/active_record/core.rb

@@ -85,7 +85,6 @@ def self.configurations
       mattr_accessor :dump_schema_after_migration, instance_writer: false
       self.dump_schema_after_migration = true
 
-      # :nodoc:
       mattr_accessor :maintain_test_schema, instance_accessor: false
 
       def self.disable_implicit_join_references=(value)