filling in some gaps in the documentation [ci skip]

Adding more information about the finders that are implemented to the docs and providing some of the migration information provided by @vipulnsward in https://github.com/rails/rails/pull/12015 to the documentation for activerecord-deprecated_finders in the docs that is now provided in the [edge upgrade guide](http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html) but still targeted at those transitioning to 4.0.