Only nullify persisted has_one target associations

add Active Job section to instrumentation guide [ci skip]

Get rid of mocha tests in actionpack - part 1

Small fixes [ci skip]

[ci skip]

This example is dependent on the `:end_at` option and has been removed
from `4-2-stable` (d167b811)

Since after 87d1aba3 `dependent: :destroy` callbacks on has_one
assocations run *after* destroy, it is possible that a nullification is
attempted on an already destroyed target:

    class Car < ActiveRecord::Base
      has_one :engine, dependent: :nullify
    end

    class Engine < ActiveRecord::Base
      belongs_to :car, dependent: :destroy
    end

    > car = Car.create!
    > engine = Engine.create!(car: car)
    > engine.destroy! # => ActiveRecord::ActiveRecordError: cannot update a
    >   destroyed record

In the above case, `engine.destroy!` deletes `engine` and *then* triggers the
deletion of `car`, which in turn triggers a nullification of `engine.car_id`.
However, `engine` is already destroyed at that point.

Fixes #21223.

Removed Mocha from ActionView part 1

Conflicts:
	guides/source/security.md

This reverts commit 9876a169.

Reason: Oxford commas are preferred. This is now documented in
the documentation guidelines.

[ci skip] Fix the grammar

This is discussed on https://github.com/rails/rails/pull/21334

* fix command list for thread
* add omitted end of code
* fix description for `continue` and `finish` command

we already have a request, so we should use the methods on the request
to access the path info information

Creates fewer request objects and helps to abstract away from internals

try to remove dependencies on `@env` so we can have more flexible
internals

Remove test helper method to inside test method

Another place to use a request object in NullSessionHash

Remove a link to the site that seems to be gone. [ci skip]

Added docs for CollectionProxy#take 

Followup to #20637.

Remove `MigrationTest#connection` and write `ActiveRecord::Base.connection`
directly to test, because `MigrationTest#connection` is only used in
`test_migration_instance_has_connection`.

May be missed in 5fe14163 commit
Also fixes the broken build

fix syntax error in strip_links example [ci skip]

can not use double quotes in double quoted string without escape

Remove not used a block argument (`&block`)

[ci skip] Add style guide about comma

This commit allows us to use one request object rather than allocating
multiple request objects to deal with the session.

We're going to implement storing env values differently in the future,
so let's disconnect these methods from the instance variables