Today there are two common ways for Rails developers to force their
applications to communicate over HTTPS:

* `config.force_ssl` is a setting in environment configurations that
  enables the `ActionDispatch::SSL` middleware. With this middleware
  enabled, all HTTP communication to your application will be redirected
  to HTTPS. The middleware also takes care of other best practices by
  setting HSTS headers, upgrading all cookies to secure only, etc.
* The `force_ssl` controller method redirects HTTP requests to certain
  controllers to HTTPS.

As a consultant, I've seen many applications with misconfigured HTTPS
setups due to developers adding `force_ssl` to `ApplicationController`
and not enabling `config.force_ssl`. With this configuration, many
application requests can be served over HTTP such as assets, requests
that hit mounted engines, etc. In addition, because cookies are not
upgraded to secure only in this configuration and HSTS headers are not
set, it's possible for cookies that are meant to be secure to be sent
over HTTP.

The confusion between these two methods of forcing HTTPS is compounded
by the fact that they share an identical name. This makes finding
documentation on the "right" method confusing.

HTTPS throughout is quickly becomming table stakes for all web sites.
Sites are expected to operate over HTTPS for all communication,
sensitive or otherwise. Let's encourage use of the broader-reaching
`ActionDispatch::SSL` middleware and elminate this source of user
confusion. If, for some reason, applications need to expose certain
endpoints over HTTP they can do so by properly configuring
`config.ssl_options`.

According to #30067:

> This PR will deprecate secrets.yml* and instead adopt
> config/credentials.yml.enc to signify what these secrets are specifically
> for: Keeping API keys, database passwords, and any other integration
> credentials in one place.

[ci skip] since only comments are being edited.

Removes most mentions of secrets.secret_key_base and explains
credentials instead.

Also removes some very stale upgrade notices about Rails 3/4.

When define callbacks latest definition on the same callback/method
overwrites previous ones.

Add comma and change verb.

[ci skip]

Adds a hint that ``after_action``-callbacks are not executed when an
exception was raised in the rest of the request cycle. The
``before_action`` section mentions "If there are additional filters
scheduled to run after that filter, they are also cancelled." but this
is IMO not sufficient.

Add a link to http://api.rubyonrails.org/classes/ActionController.html in the beginning of an article.

- By default the session store will be set to cookie store with
  application name as session key.
- Older apps are not affected as they will have the session store
  initializer generated by Rails in older versions, and Rails will not
  overwrite the session store if it is already set or disabled.
- But new apps will not have the initializer, instead the session store
  will be set to cookie store by default.
- Based on comment by DHH here - https://github.com/rails/rails/issues/25181#issuecomment-222312764.

[ci skip]

Rails 5.0 default server puma web server. following commit - https://github.com/rails/rails/commit/ae48ea69

[ci skip]

Still more to do. Please assist!

`redirect_to :back` is a somewhat common pattern in Rails apps, but it
is not completely safe. There are a number of circumstances where HTTP
referrer information is not available on the request. This happens often
with bot traffic and occasionally to user traffic depending on browser
security settings.

When there is no referrer available on the request, `redirect_to :back`
will raise `ActionController::RedirectBackError`, usually resulting in
an application error.

`redirect_back` takes a required `fallback_location` keyword argument
that specifies the redirect when the referrer information is not
available.  This prevents 500 errors caused by
`ActionController::RedirectBackError`.

AbstractRequest has been deprecated, updating to refer to ActionDispatch::Request instead.

[ci skip]

typo "description not clear corrected with proper description and action_controller_overview file Rails' ->  Rails" [ci skip]

Followup to #20637.

- Changed `IN` to `ON` in all note sentences in guides.