[ci skip] Running AR tests guide [Fixes GH-15891]

Patch by @bcjordan
Follow up on #16408

[skip ci]
Merge branch 'jonatack-patch-8'
add nested_model_forms to guides/documents.yaml [wip] [ci skip]
Merge pull request #11859 from zzak/nested_model_forms_guide

See AR::Associations for greater detail in Nested Model Forms Guide [ci skip]
Pass logging message through block.

This follows the good practice listed on http://guides.rubyonrails.org/debugging_rails_applications.html#impact-of-logs-on-performance
Missing space. [CI SKIP]
Merge pull request #16634 from tgxworld/missing_space

Missing space. [CI SKIP]
Merge pull request #16633 from tgxworld/prefer_log_message_through_block

Pass logging message through block.
Using `each_with_object` instead of `reduce`

This way no new object allocation is taking place. Thanks @jeremy for
the suggestion!
add I18n support for `:placeholder` HTML option is passed to form fields
just use the placeholder tag value if it is passed as a String
Merge pull request #16560 from robin850/rbx-duration-eql

Define the Duration#instance_of? method
Merge pull request #16626 from adomokos/replacing-each-with-reduce

Replacing an each with reduce
Implemented enqueue_at for ActiveJob's Backburner adapter
Active Job guide: Add info about queue_name prefix and clarity supporter features
Roll back open transactions when the running thread is killed.
Merge pull request #16622 from matthewd/default-debug

Don't fight against ourselves for the default log level
Merge pull request #16642 from cristianbica/aj-doc-queue-name-prefix

Active Job Guide: Example of config queue_name_prefix [ci skip]
Merge pull request #16641 from cristianbica/aj-backburner-enqueue-later

Implemented enqueue_at for ActiveJob's Backburner adapter