Change wording & punctuation in Guides - Upgrading

Partially revert #25192

KeyGenerator is used in other contexts, and we cannot change its
output... even if it does accidentally default to generating excess key
material for our primary internal usage.

Publish Rails 5 release notes

Update testing guide for Rails 5

Update release notes for 5.0 [ci skip]

Update upgrade guide

- 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.

- Update the testing guide as per output of test runner.
- Show how to see all the options provided by test runner.
- We no longer generate fixture named :first.
- Update the request methods with URL helpers and also fix few wrong occurrences.
- Generated test case does not have assert_includes for response body.
- This removes code from https://github.com/rails/rails/commit/ca83436d
  but I think that might be oversight. We don't generate such code in
  the generated controller test.
- Update the helper example to suite the integration style of tests.
- We can't modify session directly in new integration style controller
  tests.
- Show an example which matches with integration style request tests.
- Active Job default adapter for tests is async, not test.
- Fix the example of request headers and CGI variables.

Update getting started guide for Rails 5 [ci skip]

- Rails 5 will return 204 No Content by default for controller actions
  which do not have a template or do not specify how to render.
- The "Getting started" guide needs to be updated for this.

This is an slight refactoring/improved version of #25586.

add missing things to release notes [ci skip]

Add following.

* https://github.com/rails/rails/commit/05934d24aff62d66fc62621aa38dae6456e276be
* https://github.com/rails/rails/commit/38d2bf5fd1f3e014f2397898d371c339baa627b1
* https://github.com/rails/rails/pull/25469

Add API Apps to release notes

Pass over Upgrading guide

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

Make release notes about legacy `mysql` adapter clearer.

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

Add documentation about Action Cable npm package

[ci skip] Update formatting in ActionView::Base documentation

[ci skip] Fix grammar & add link in PolymorphicRoutes

Do not inspect array of over 10 elements

Do not specal case inspecting associated arrays of over 10 elements, preventing infinite looping in some cases.

Right now with this middleware we are likely always connected, but I'm
hoping to change that soon. This is an easy change to pull out on its
own.

This method appears to have been partially used in connection pool
caching, but it was introduced without much reasoning or any tests. One
edge case test was added later on, but it was focused on implementation
details. This method is no longer used outside of tests, and as such is
removed.

update description of `test_order` [ci skip]