[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.
Extract join values to merge as an original type

```ActiveRecord::QueryMethods#joins!``` receives arguments as variable
length arguments. So that it saves join values as Array.

This affects ```ActiveRecord::Relation::Merger#merge_joins``` which
handles joins by its type. If String joins merged twice, the second
merge process raises ```ActiveRecord::ConfigurationError```