- Also to_time_preserves_timezone config should be false for older apps getting upgraded to Rails 5 [ci skip]

Mentions missing config options and fixes headers for section talking
about new framework defaults.

It's useful to have a link to the release notes in the upgrade guide for
a clearer view of what changed in Rails.

Adds missing upgrade items to the upgrade guides for Rails 5. Fixes some
typos and whitespace as well.

Ruby 2.4 unifies Fixnum and Bignum into Integer: https://bugs.ruby-lang.org/issues/12005

* Forward compat with new unified Integer class in Ruby 2.4+.
* Backward compat with separate Fixnum/Bignum in Ruby 2.2 & 2.3.
* Drops needless Fixnum distinction in docs, preferring Integer.

[ci skip]

- Change from "The Task" to "The Update Task" as referred elsewhere

[ci skip]

`rails:update` was deprecated in 6fb31638

This guides were pointing to this command `rails app:update`, which I
tried to run, but it didnt worked. I think the right command is `rails
rails:update` instead. Also thats the name of the rake task.

Also I removed the `Rake` word from the title, as we run it using
`rails` bin now.

cc @kaspth
[skip ci]

(e.g. `rails:update` and `rails:template` tasks is renamed to `app:update` and `app:template`.)

[ci skip]

The iterative process one has to follow is not currently documented, and
people could be unfamiliar with the Rails versioning scheme, causing
them to skip versions or upgrade to patch releases.

[ci skip]

xref #22701.
xref #20612.

[ci skip]

The previous title was misleading.

[ci skip]

Should use `public_file_server.enabled` instead.

Clarified that static files will be served from the public directory, where it made sense.

Also removed occurrence of the deprecated `static_cache_control`.

Move from `AS::Callbacks::CallbackChain.halt_and_display_warning_on_return_false`
to `AS::Callbacks.halt_and_display_warning_on_return_false` base on
[this
discussion](https://github.com/rails/rails/pull/21218#discussion_r39354580)

Fix the documentation broken by 0a120a81

Ref #21618

[ci skip]

The has_and_belongs_to_many default join_table behavior changed
between Rails 3.2 and Rails 4.0 and should be noted in the
appropriate section of the upgrade guide.

commit:
https://github.com/rails/rails/commit/46492949b8c09f99db78b9f7a02d039e7bc6a702