[ci skip] Fix typo in command_line.md

[ci skip] Fix comment of SourceAnnotationExtractor#find_in

Commit(810af6f6) changed which extensions are taken into account,
so make to match comment of find_in.
And sort extensions to follow the added order.

[ci skip] Fix line break on command line guide

default scopes should break the cache on singulur_association.

Add a section about job testing [ci skip]

fixes #17495

Delegate comparison operator to value

if you specify a default scope on a model, it will break caching.  We
cannot predict what will happen inside the scope, so play it safe for
now.  fixes #17495

Guides: Corrected name for assigns key [ci skip]

- Integration test example is now using 'articles' name
  for both path and assigns key.

[ci skip] modify default host in command line guide

Print out a meaningful error when ActiveRecord::ReadOnlyRecord is raised

Currently, there is no messages which get printed out. Convoluted system
may have hooks that create other objects in which case we only fail with
no messages. This commit changes this information allowing you to know
which object is the one that actually raised the error.

[Enh] Made the ActiveModel::Dirty#clear_attribute_changes method public

In Rails 4.2 it is impossible to define a custom default value for a model's
attribute without making it appear as _changed?, especially when the model
is first initialized. Making this method publicly visible will allow such a behaviour,
without the need to use private APIs.

added example of hash#except, and removed extra whitespaces [ci skip]

This makes debugging the generated schema output much easier.
As a side effect it also shaves off 2.5 seconds of test runtime.

Remove session to allow `with_routing` to be called twice.

Fixed grouped expression warning

Update docs 6

/cc @chancancode

Remove redundancy [ci skip]

Replace Html with HTML [ci skip]

pattern removal

added example for string#remove and test case for remove of multiple occurence of pattern

removed extra whitespaces

- Everywhere else we using HTML Sanitizer except this place.

I love this guide! 
and "visually see" is redundant.
remove "visually", then the sentence reads:
> Action Mailer previews provide a way to see how emails look

[docs] Removed respond_with documentation as it was deprecated and removed from Rails [ci-skip]

of respond_to. respond_with was moved into the responders gem and deprecated
inside rails, so there is no need to mention it within rails itself.