Correct test description for large integer test
[Testing guide] Use "Fixtures API" instead of "fixture api".
[Testing guide] Using "Fixtures API" consistently

- In guides, "API" is used consistently instead of "api".
- Reason behind replacing "fixtures API" to "Fixtures API" is because
  here we are referring to the concept of "fixtures". In other places in
  the guide "fixture" OR "fixtures" is referred to the single or
  multiple instances of test data.
- This is also followup of https://github.com/rails/docrails/commit/aa9d1332b20305dd860b21a7ada06c5252bebad5#commitcomment-8497376.
Fix grammar on ActiveRecord::AttributeMethods doc.

[ci skip]
allow 'all' for :domain option in addition to :all
Don't remove mailer layouts files
Refactor `SchemaCreation#visit_AddColumn`
updates some pointers in the guides welcome section [ci skip]
StatementCache is private API

It should not be used in applications
Action Mailer change log pass

[skip ci]
Update the StatementCache documentation
Merge pull request #17800 from jonatack/patch-8

Action Mailer change log pass [ci skip]
Fix CHANGELOG typo introduced in e2cce6cd [ci skip]
Follow-up to e2cce6c

[skip ci]
Merge pull request #17801 from jonatack/patch-8

Clarification follow-up to e2cce6c [ci skip]
[ci skip] Remove outdate note of :dependent
Merge pull request #17802 from aar0nTw/remove-dependent-with-through-note

[ci skip] Remove outdate note of :dependent in guide
Merge pull request #17803 from sadfuzzy/patch-2

Update cookies.rb
Conflicts:
	actionpack/lib/action_dispatch/middleware/cookies.rb
Revert "Test against rack master"

This reverts commit 2a4e14db981e38611667d407a975600ee720ada7.
Bump to rack 1.6.0.beta2