Add propery docs to ActionDispatch::Response [ci skip]

After some discussion on Twitter with @skud, the documentation on ActionDispatch::Response is
a bit sparse. This class is useful when you're writing tests, as often you want to assert various
things about the response that's coming back. Better docs would make this easier for people new
to testing in Rails.

I only added some descriptions for various properties that were defined, and mostly just a sentence
or two. Most of these things are familliar if you're working with HTTP, but some words is better
than no words at all.

Hopefully further commits will fix up things that aren't just documentation.
rewrites AR's reload documentation

* It is no longer true that options are passed to find.

* Documents that ActiveRecord::RecordNotFound may be raised.

* Documents that the reload happens in-place.

* Documents that caches are wiped.

* Documents that the method returns self.

* Documents a couple of use cases.
revises the documentation of ActiveRecord::Relation#find_or_create_by [ci skip]

* Inspect uses double quotes.

* Inspect puts a hash as in #<User ...>.

* Documents the return value, and makes explicit it can be an invalid record.

* Documents the method is not atomic.

* Documents a way to handle UNIQUE contraint violations in the event of a race condition.

* Removes the "Examples" header according to our guidelines.
Remove active_support/json/variable was deprecated.
Remove ActiveSupport::Benchmarkable#silence was deprecated.
Remove active_support/testing/pending.rb was deprecated.
Remove :confirm and :disable_with options for ActionView::Helpers::UrlHelper#link_to and #button_to were deprecated.
Remove link_to_function / button_to_function from ActionView::Helpers::JavascriptHelper were deprecated.
Remove :confirm and :disable_with options from ActionView::Helpers::FormTagHelper were deprecated.
Remove action_controller/vendor/html-scanner.rb was deprecated.
Remove ActionController::RecordIdentifier was deprecated.
Remove #sum with a block was deprecated.
Remove ActionView::Template#mime_type was deprecated.
Merge pull request #10814 from kennyj/deprecations_4_1

Remove some deprecation methods / options / files.
Remove Mime::Type#verify_request? and Mime::Type.browser_generated_types were deprecated.
Raise ArgumentError to fix Rails 4.1 deprecation warn message.
Remove behavior that deals with a string as third argument of `add_index`, because this was deprecated.

Some testcases is failed, so I replaced nil to empty hash in add_reference.
Remove instance level attr_readonly setting was deprecated.
Remove instance level serialized_attributes setting was deprecated.
Remove unnecessary testcase for instance level stored_attributes.