asset guide, match application.rb snippets to the generated file [ci skip]

`if defined?(Bundler)` is no longer used in our current application.rb
Document ActionMailer::Base#set_content_type
Adding notes on PATCH to the upgrade guide.

Discusses compatibility between PATCH and PUT, as well as how to add
support for JSON PATCH to your application.

Fixes #10439.
emphasize that callbacks are called in destroy_all

Cleaned up rdoc a bit emphasizing that callbacks are called. Also
removed the stress on the fact that records are always removed.

If callbacks return false then records will not be deleted.
fix grammar
Added documentation for Rails::Generators::NamedBase.template
Added documentation for model migration generation
improved doc for ActiveRecord#find_by_sql method (Refs #10599) [ci skip]
Use new hash syntax
ActionMailer::Collector should be nodoc'd
Document ActionMailer::LogSubscriber and ActionMailer::DeliveryMethods::ClassMethods
Document ActionMailer::MailHelper
nodoc ActionMailer module from lib/action_mailer/*.rb
Document Rails::Generators::MailerGenerator
Added documentation for ActiveRecord::Associations::Builder::Association class
Added some more documentation for ActiveRecord::Associations::Builder::Association class
Added some more documentation for define_readers and define_writer of the Association and its inherited classes
Corrected documentation and added some more for the classify method in inflectors
Corrected documentation regarding validation errors
Merge pull request #137 from zduci/master

Corrected documentation regarding validation errors