Clarify config settings for AS::halt_callback_chains_on_return_false

[jonatack]

Despite the config name, it isn't super clear when to use true or false for this config and which of the two is the new default. I remember searching in the docs to make sure I got it right. Clarifying it here might help others to be sure they got it right, too.

[skip ci]

[rails-bot]

r? @sgrif

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[sgrif]

Can we just add docs for the setting instead? I don't think we need to exhaustively document config options in the config file itself.

[jonatack]

Moved docs from the config file to the Configuring Guide.

[jonatack]

The generated config file defaults to false, but if no setting is given the default is true as per the change log reproduced below. Will update the docs to clarify that.

Newly generated Rails apps have a new initializer called
`callback_terminator.rb` which sets the value of the configuration option
`ActiveSupport.halt_callback_chains_on_return_false` to `false`.

As a result, new Rails apps do not halt Active Record and Active Model
callback chains when a callback returns `false`; only when they are
explicitly halted with `throw(:abort)`.

The terminator is *not* added when running `rake rails:update`, so returning
`false` will still work on old apps ported to Rails 5, displaying a
deprecation warning to prompt users to update their code to the new syntax.

[claudiob]

Your latest comment looks good 🎀

[matthewd]

This seems like quite a story, particularly given that the only people who see this (new 5.0 apps) really shouldn't care about it at all.

Have we previously been this verbose on similar config values?

(This is more of a general observation about the comment as it already stood, unrelated to your specific changes.)

[jonatack]

There is the same addition in active_record_belongs_to_required_by_default.rb.

This change could be removed from this PR and added to another one for both config files.

[jonatack]

The changes were in 569e03d

[jonatack]

@claudiob I hesitate because technically, the default is currently just true during the deprecation period and will switch to false afterward. FWIW your change log rocks.

[claudiob]

Ok then fine with closing it 😁

[rafaelfranca]

Closing them.

[jonatack]

I just updated the PR. The configuration still needs to be explained in the docs.