remove nonexistent hook from instrumentation guide [ci skip]

Update rails_welcome.png

[ci skip]

[ci skip] Make clear what `this method` is

In this sentence, there are three candidates of `this method`

* default_url_options
* url_for
* helper method(s)

so make it clear.

As of 56903585, the Rack::ContentLength middleware isn't included by
default anymore.

The only remaining middleware is Rack::Lock ; since it's only included
when using WEBrick, it's certainly not worth documenting it in the
guides.

[ci skip]

[ci skip] correct for ActiveRecord::Associations::Preloader

Fix `undefined method uncached` for polymorphic belongs_to #20426

thrown ActiveRecord::AssociationTypeMismatch when assigning a wrong value for a namespaced association

do not show "Failed tests" message when a failed test is not

Remove mistaken end from controller_path doc [ci skip]

Respect routing precedence for HEAD requests

Add nodoc to the Validations::Helpers [ci skip]

Minor fixes [ci skip]

thrown ActiveRecord::AssociationTypeMismatch when assigning a wrong value for a namespaced association

fixes #20541

The heading "Active Model Length Validator" was shown on the
"ActiveModel::Validations" page without any text following it.

This is a small refactoring that simplifies the Active Record specific
lenght validator.

Associated objects that were marked for destruction are considered absent.

Move the validations HelperMethods to its own file

Fix the documentation about ActionController::UnpermittedParameters [ci skip]

remove unused variable in reporter test

Fix the documentation about ActionController::ParameterMissing [ci skip]

Remove duplicate private statement

Closes #11209

[Roque Pinel & Steven Yang]

Conflicts:
	guides/source/configuring.md

[ci skip]