Add a note explaining when the has_many associations callbacks will be called or not.

That is happened with `distinct`, and that is not only happened on MyS
L, but also on PostgreSQL, SQL Server, and Oracle.

[ci skip]

In e8546aba, the Getting Started guide
was updated to reflect the removal of CoffeeScript stubs being generated
by the controller generator. However, the comment above the table of
generated files wasn't updated, so it incorrectly remained "This creates
five files and one empty directory". This fixes it to now say "This
creates four files and one empty directory".

This reverts commit 6b69ae51, reversing
changes made to 84bd9ada.

Reason: We already use "happen" a lots for callbacks and validations.

https://github.com/rails/rails/pull/35324#issuecomment-465096483

[ci skip]

Change 'happen' to a more technical term

[ci skip]

This reverts commit b46601b5, reversing
changes made to 4e6737f1.

This reverts commit 141f50ef, reversing
changes made to 317efa5c.

Reason: `add_foreign_key` and `remove_foreign_key` are implemented at
#35212.

[ci skip]

Swap `#users` jQuery selector to correct position and prevent the escaping of HTML.

If they're not set we'll still fall back to localhost, but this makes it
possible to run the tests against a remote Postgres / Redis / whatever.

[ci skip]

Changes the Getting Started guide explanation for
`ActionController::MissingExactTemplate` error, to reflect the current message.

Follow up for #29286, #35148

[ci skip]

This commit updates the Missing Template image in the Getting Started
Guide.

The new image displays the current error message. Additionally:
- New image is a PNG file, whereas the current image is in fact a misnamed JPEG.
- New image is smaller.

Before:

```
$ file --mime -b template_is_missing_articles_new.png
image/jpeg; charset=binary
$ du -h template_is_missing_articles_new.png
464K    template_is_missing_articles_new.png
```

After:

```
$ file --mime -b template_is_missing_articles_new.png
image/png; charset=binary
$ du -h template_is_missing_articles_new.png
28K     template_is_missing_articles_new.png
```

[ci-skip]

During the development of #33145, I have named a few concepts in the
code as `whitelisted`. We decided to stay away from the term and I
adjusted most of the code afterwards, but here are the cases I forgot to
change.

I also found a case in the API guide that we could have cleaned up as
well.

[ci skip]

[ci skip]

The AV::Base constructor was too complicated, and this commit tightens
up the parameters it will take.  At runtime, AV::Base is most commonly
constructed here:

  https://github.com/rails/rails/blob/94d54fa4ab641a0ddeb173409cb41cc5becc02a9/actionview/lib/action_view/rendering.rb#L72-L74

This provides an AV::Renderer instance, a hash of assignments, and a
controller instance.  Since this is the common case for construction, we
should remove logic from the constructor that handles other cases.  This
commit introduces special constructors for those other cases.
Interestingly, most code paths that construct AV::Base "strangely" are
tests.

Follow-up to #35055.

and suggest a workaround

Don't upsize images smaller than the specified dimensions.