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.