Expand AR::Base.abstract_class documentation
Expand AR::Base.abstract_class documentation

The previous documentation is somewhat unclear about the use case for an
abstract ActiveRecord class.

This clears it up by highlighting the following points:

  - table_name is not derived from the abstract class' name
  - type is not derived on direct descendants of the abstract class
  - validations, not abstract_class, should be used to specify whether
  the parent model can be instantiated or not

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