Improving Method Chaining section [ci skip]

assert template should raise ArgumentError for unsupported layout types

"backoffice" -> "back office", "lookup" -> "look up" [ci skip]

"Lookup" is a noun, but not a verb. The verb is "look up".

[skip ci]

Add a code checking about file or not to the rails db command

allow reseting of request variants

Add test for ActiveModel `after_validation`, `after_` and `around_` callbacks returning false

Test to ensure content_tag works when fixing #17661

Add test for `:skip_after_callbacks_if_terminated`

After c2fe0938, which was reverted yesterday, there will likely be
future PRs to address #17661.

The test `actionview/test/template/capture_helper_test.rb
#test_capture_returns_nil_if_the_returned_value_is_not_a_string` should
have errored with c2fe0938, but it was rewritten in the PR to not raise.
My guess is that it may have seemed irrelevant to the content_tag tests
or already covered by them.

This test provides additional protection by being in the content_tag
test suite to explicitly raise a red flag in future cases. It foregoes
some redundancy for safety — at least until #17661 is closed.

Improve section in constant autoload guide

Remove misleading test: around_action return false

English fix [ci skip]

Improve grammar

[skip ci]

The current implementation of `variants=` don't allow a resetting to nil, wich is the default value.

This results in the following code smell:
```ruby
case request.user_agent
when /iPhone/
  request.variants = :phone
when /iPad/
  request.variants = :ipad
end
```

With the ability to reset variants to nil, it could be:
```ruby
request.variants = case request.user_agent
when /iPhone/
  :phone
when /iPad/
  :ipad
end
```

Constant autoload grammar fix

[skip ci]

[skip ci]

[ci skip] ✂ blank lines between list items.

[ci skip]

/cc @sgrif @zzak

Conflicts:
	activerecord/CHANGELOG.md

I think "What" is was fine, "The purpose of" assumes the reader knows such thing exists.
"What is" is fine, and also matches the following "What is require_dependency".

4.2.0", then they should have no problems hacking with Rails. [ci skip]

If they do, then its a bug and it should be reported.

Improving tips about installation in the guides [ci skip]

[ci skip]

Conflicts:
	guides/source/constant_autoloading_and_reloading.md

Counter cache Rails guides fix

Adding method chaining section on ActiveRecord querying guide [ci skip]

[ci skip]

Improve autoload_paths section [ci skip]

You need to specify the counter_cache option on the has_many side of the association when using a custom counter cache column. This is documented on the has_many association here: http://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_many [ci skip]