Refs #17684: Enhances rdoc for ActiveJob assert_performed assertions

It will set proper queue_adapter state required by assert_performed_jobs assertions.

Also the block version of assert_performed_jobs will make sure this
state is respected.

Fixes #17684

Fixes https://github.com/rails/rails/issues/16958

[Byron Bischoff & Melanie Gilman]

Follow up to #17646.

Creating mailer layouts by default, including html and body tags

This allows us so abstract the migration from the type that is actually
used by Rails.  For example, ":string" may be a varchar or something,
but the framework does that translation, and the app shouldn't need to
know.

Explain that default_scope also influences update_all

Use sprockets-rails 2.2.x on base app and allow greater version

- Remove sprockets-rails from generated Gemfile as rails has a hard-dependency on it

- Also allow sprockets-rails >= 2.0.0

It should be part of the documented public API, since we have an entire
section of the guides dedicated to it. Documented in a way that
addresses the concerns which kept it undocumented in the past.

Show source view and backtrace on missing template errors

This will help you debug missing template errors, especially if they
come from a programmatic template selection. Thanks to @dhh for
suggesting that.

As a bonus, also show request and response info on the routing error
page for consistency.

Mark comments that should not be in the docs [ci skip]

Some comments that are meant to separate blocks of code in a file show up
on http://api.rubyonrails.org as though they were part of the documentation.

This commit hides those comments from the documentation.

Stems from the discussion with @zzak at https://github.com/voloko/sdoc/issues/79#issuecomment-64158738

[ci skip]

[related #17718]

Thanks to @thedarkone for pointing out that an instance of this object
is used in a shared context.

Deprecate use route

Replace ActionDispatch::Head with Rack::Head.

Fix includes on association with a scope

Move test that depends on ActiveRecord

Fix wrong path in comments about stylesheets

I put the wrong path in my last PR by accident. Fixed here. Related to #17742

Change 'of plugins' syntax

Currently, the docs uses a syntax that is unclear and not general
American English. I've switched it to be clearer wording. Not a big
fix, but may be helpful.

It now contains a carefully formulated reference to the "current relation" which might help clarify that the receiving will generate its own scope, escaping the need for explicitly referencing `default_scope` which is, after all, just another way of specifying a scope and nothing special.