fix link in autoloading guide [ci skip]

In the case of serialized columns, we would expect the unserialized
value as input, not the serialized value. The original issue which made
this distinction, #14163, introduced a bug. If you passed serialized
input to the method, it would double serialize when it was sent to the
database. You would see the wrong input upon reloading, or get an error
if you had a specific type on the serialized column.

To put it another way, `update_column` is a special case of
`update_all`, which would take `['a']` and not `['a'].to_yaml`, but you
would not pass data from `params` to it.

Fixes #18037

Add class level case operator support for error dispatching in Rescuable

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