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]
fixes default attributes for button_tag
fix accept nil options
fix button tag without options
fix form button
fix indentations
Add a comment to ensure that a test won't be removed [ci skip]

The 'cow' => 'kine' inflection has gone with c300dca9 but it should
not be removed from the tested irregularities since it ensures that
inflections work with words that do not begin with the same letters.
fix content_or_options to be replaced by options and set to nil
Add spaces to deep_munge log message.
cleanup and move extracted method right into the helper
Merge pull request #14258 from shuhei/fix_deep_munge_log

Add spaces to the log message of ActionController:: LogSubscriber#deep_munge
rollback to private method
Cache key should be different when is Array.

`cache.fetch(['foo'])` and `cache.fetch('foo')` should generate
different cache keys as they are not equivalents.

[related #8615]
[related #8614]
Replace map.flatten with flat_map in actionpack
Replace map.flatten with flat_map in activerecord