typos fixes
Tweak Configuring Guide language

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

This commit also addresses rails/docrails#169 and rails/rails#14159
Improve guide for Hash#transform_keys and related methods.

[skip ci]
Fix code alignment in duplicable? guide.

[skip ci]
Improve collision explanation for Hash#transform_keys.

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

[skip ci]
Do note remove `Content-Type` when `render :body`

`render :body` should just not set the `Content-Type` header. By
removing the header, it breaks the compatibility with other parts.

After this commit, `render :body` will returns `text/html` content type,
sets by default from `ActionDispatch::Response`, and it will preserve
the overridden content type if you override it.

Fixes #14197, #14238

This partially reverts commit 3047376870d4a7adc7ff15c3cb4852e073c8f1da.
Update doc for TagHelper.
[ci skip] Add "Low-Level Caching" part to "Caching With Rails"
Merge pull request #14285 from sikachu/master-revert-render-body-remove-content-type

Do note remove `Content-Type` when `render :body`
Merge pull request #14257 from killthekitten/patch-1

Add "Low-Level Caching" part to "Caching With Rails" guide
unscope doesn't remove only the default_scope, but all of them.

[ci-skip]
Closes rails/rails#14294
fix typo app -> all

Thank you @bquorning
Merge branch 'master' of github.com:rails/docrails

Conflicts:
	guides/source/4_1_release_notes.md
Add preventDefault() on click event
Merge pull request #14316 from the4dpatrick/master

Add preventDefault() on click event