Cleaning up ActiveModel::Dirty tests

* Clarifying what the #changed method returns
* Adding tests to describe what the #changed_attributes returns
* Updating test name based on pull request comment
* Moving the test lower in the file per pull request comment
Update guides/source/active_record_validations.md

Duplicated `update`
Merge pull request #8940 from adomokos/adding_tests_for_changed_attributes

Cleaning up ActiveModel::Dirty tests
Merge pull request #8954 from davetoxa/patch-1

Update guides/source/active_record_validations.md