Updated the guides for having a distinct has_many through at the database level
Updated the guides for having a distinct has_many through at the database level

The current example

    add_index :person_articles, :article, unique: true

Does not work, the `:article` column does not exist as it will be
a join table (based on the prior example) so should use :article_id

The documentation seems to suggest that it will allow an article to
be added only once to a person via the join table, what actually
occurs is that it only allows the article to be added to one person,
at which point it should be a `belongs_to` association.

Also changed the new example to use readings based on the prior example

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