docs, make association `autosave: true` examples runnable. Closes #14700

[ci skip]

The examples are written in a way you expect them to be executable.
However one snippet assumed there to be two comments when only one
was created above.

The defined models did not extend `ActiveRecord::Base`

The example used `comments.last.mark_for_destruction`. This does no
longer load the whole collection but just the last record. It is
then refetcht on subsequent calls to `last`. This breaks the example.