Merge pull request #12232 from claudiob/delete-unused-helper-fixtures

Remove unused AV helper fixtures from e10a2531
return a list rather than hash
combine methods so we can reuse preloaders
cache associated target records hash
Removes redundant into text

Looks like a remnant sentence fragment from the 3.2 guide.
[ci skip] Add some comment about downcase url encoded string.
[ci skip] Fix a typo in Engines.md.
Merge pull request #12310 from JuanitoFatas/fix-a-typo

[ci skip] Fix a typo in Engines.md.
Merge pull request #12307 from douglascalhoun/patch-1

Removes (redundant?) sentence fragment in intro text
Fix typo. This test isn't executed in even postgresql.
Add back options argument in the ActiveRecord::Base.initialize method

This will make easier to hook protected_attributes gem in our code
without making that gem fragile to change in Rails code base.

Closes #12243
Merge pull request #12311 from kennyj/fix_typo_about_uuid

Fix typo. This test isn't executed even in the postgresql.
[ci skip] Add a type modifier in migrations.md.
[ci skip] Add missing migrate step in generating comment resource section.
Add notes about database connection pool [ci skip]
Merge pull request #12316 from JuanitoFatas/engines

[ci skip] Add missing migrate step in generating comment resource section.
Merge pull request #12281 from kennyj/add_doc_about_12276

[ci skip] Add some comment about downcase url encoded string.
Merge pull request #10500 from kennyj/fix_10450

Fixed a bug in  when using has_many association with :inverse_of option and UUID primary key.

Conflicts:
	activerecord/CHANGELOG.md
Give the credits [ci skip]
use active voice and use more descriptive terms