additional note about routes in functional tests for engines [ci skip]
Improve Contributing to Rails Guide

Logically group the content, so it makes more sense if someone tries to read from start of page to end of page. [ci skip]
Add documentation for Thread#freeze
Instead boat load of libraries, simply say StdLib
Use path where extensions are defined, not used

For consistency with all other documentation, point to where these core
extensions are defined, not where the example code comes from.

[ci skip]
Document LoadError core extensions

[ci skip]
Fix a few grammar issues. [ci skip]
Correct docs as singlton true is not returned here

[ci skip]
Remove surplus period from assertion messages
Fix spelling errors and grammar issues

Including replacing a few "smart quotes" with regular quotes (' and "),
for consistency.
[ci skip]
[ci skip] Improve doc, fix grammatical issue
Display diagnostics in text format for xhr request
Refine tests for assert_select failure messages
Fix subscriptions not being unsubscribed.
Remove wrapping div with inline styles for hidden form fields.

We are dropping HTML 4.01 and XHTML strict compliance since input
tags directly inside a form are valid HTML5, and the absense of
inline styles help in validating for Content Security Policy.
Use inheritance chain instead of callbacks to increment counter caches after create
Use inheritance chain instead of callbacks to increment counter caches after destroy
Merge pull request #14729 from akshay-vishnoi/doc_changes

[ci skip] Improve doc, fix grammatical issue
Remove outdated comment
Return null type format when format is not know

When requesting a controller with the following code with a unknown format:

    def my_action
      respond_to do |format|
        format.json { head :ok }
        format.any { render text: 'Default response' }
      end
    end

we should render the default response instead of raising ActionController::UnknownFormat

Fixes #14462

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/test/controller/mime/respond_with_test.rb

Conflicts:
	actionpack/CHANGELOG.md