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]
Update doc for TagHelper.
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