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.
Merge pull request #10821 from kennyj/deprecations_4_1-2

Remove some deprecated methods in Rails 4.1 again.