Add :method docs for ActiveRecord::AttributeMethods::Dirty dynamic methods

[nhorton]

ActiveModel::Dirty has doc entries like https://api.rubyonrails.org/classes/ActiveModel/Dirty.html#method-i-2A_changed-3F for the dynamically-generated methods. https://api.rubyonrails.org/classes/ActiveRecord/AttributeMethods/Dirty.html does not have such docs, and they would be very helpful.

There is a correlated issue that the ActiveModel docs come up in web searches WAY higher than the ActiveRecord ones. I would suggest two changes to help with this for SEO and discoverability.

1. Reference the ActiveRecord doc in the ActiveModel one so folks know it exists if they end up on ActiveModel's
2. Add a small section on dirty tracking to one of the ActiveRecord Rails Guides. This would be a great thing in general as it is massively useful and many people don't know about it.