Based on comments at
https://github.com/rails/docrails/commit/42417b0013819e573be9ffc5402e179003156e8e#commitcomment-5527251

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This commit also addresses rails/docrails#169 and rails/rails#14159

the fact that Rails is multi-thread by default is not very relevant. see https://github.com/rails/rails/pull/13998 [ci skip]

change 'assert !' to 'assert_not' in guides [ci skip] 

Add note to silence_stream and quietly. [skip ci]

State that on the doc.

[fixes #11954]

Close the meta tag with '/>' instead of '>'

This change fixes the 42 instances of `end tag for "meta" omitted, but
OMITTAG NO was specified` error message that comes up when the full
railsguides are validated.

Full error list at
https://gist.github.com/prakashmurthy/9086679.

The errors were introduced in e220a34e

Update Docs in favor to use render plain instead of text option

ref #14062

Typo fix for unscope [ci skip]

Before we were calling to_sym in the mime type, even when it is unknown
what can cause denial of service since symbols are not removed by the
garbage collector.

Fixes: CVE-2014-0082

Conflicts:
	actionview/CHANGELOG.md
	activerecord/CHANGELOG.md

Thanks Godfrey Chan for reporting this!

Fixes: CVE-2014-0080

Previously the values of these options were trusted leading to
potential XSS vulnerabilities.

Fixes: CVE-2014-0081

Sync 4.1 release notes with changes since 7f648bc7

Introduce `:plain`, `:html`, and `:body` render options.

Setting this attribute to `true` will remove the content type header
from the request. This is use in `render :body` feature.

* Introduces `:plain`, `:html`, `:body` render option.
* Update guide to use `render :plain` instead of `render :text`.

This test were assuming that the list of render options will always be
the same. Fixing that so this doesn't break when we add/remove render
option in the future.

This is an option for to HTML content with a content type of
`text/html`. This rendering option calls `ERB::Util.html_escape`
internally to escape unsafe HTML string, so you will have to mark your
string as html safe if you have any HTML tag in it.

Please see #12374 for more detail.

This is as an option to render content with a content type of
`text/plain`. This is the preferred option if you are planning to render
a plain text content.

Please see #12374 for more detail.

This is an option for sending a raw content back to browser. Note that
this rendering option will unset the default content type and does not
include "Content-Type" header back in the response.

You should only use this option if you are expecting the "Content-Type"
header to not be set. More information on "Content-Type" header can be
found on RFC 2616, section 7.2.1.

Please see #12374 for more detail.

Document `default_scope` changes

[ci skip]