[ci skip] Update plugin doc with the latest rails
Removed unnecessary word
[ci skip] config.active_record.errors_in_transactional_callbacks -> config.active_record.raise_in_transactional_callbacks
Merge pull request #16553 from tomkadwill/removed_word

[ci skip] Removed unnecessary word from Getting Started
http://gembundler.com/  =>  http://bundler.io/
4.2 Release Notes pass [skip ci]
Merge pull request #16552 from untidy-hair/update_plugin_guide

[ci skip] Update plugin doc with the latest rails
use same idiom to define class method as in every other adapter.
"warning: assigned but unused variable"
Merge pull request #16554 from y-yagi/patch-2

[ci skip] config.active_record.errors_in_transactional_callbacks -> config.active_record.raise_in_transactional_callbacks
Merge pull request #16556 from jonatack/patch-9

4.2 Release Notes pass [ci skip]
Allow sending all emails through activejob

Added config.action_mailer.deliver_all_emails_later boolean flag and #deliver_now / #deliver_now!
methods. When #deliver / #deliver! is called, based on the value of deliver_all_emails_later
it will call #deliver_now / #deliver_now! or #deliver_later / #deliver_later!.