Describe how has_many's :dependent option affects #delete

I was puzzled about why `collection=` was destroying the removed
records on an association with `dependent: :destroy`, even after
consulting the documentation for that option.  I had to dive into the
Active Record source to understand what was going on: eventually
`collection=` calls `collection.delete` on the ousted records, and it
also uses the `:dependent` option to decide how to remove records.

It would have helped me to have mention of this in the documentation for
`:dependent`, not just under `collection.delete` (which I found much
later).

Briefly mention the broader impacts of `:dependent` in the Association
Basics guide.

[ci skip]

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