Add ".[key]" to the abstract namespaces [ci skip] (rails#50860)

* Add ".[key]" to the abstract namespaces [ci skip]

The section "Error Message Scopes" describes how the Rails look up the
namespaces for a validation error message with code snippets.
The first code snippet shows abstract namespaces like
`errors.attributes.[attribute_name]` while the second one demonstrates
the materialized one like `errors.attributes.name.blank`.
The first code snippet lacks the validation error "key." My team member
was confused and created a translation without "key", so I think it
would be helpful to add ".[key]" to the abstract namespaces.

* Move abstract key list after :blank examples

Co-authored-by: Rafael Mendonça França <[email protected]>

Author: yykamei

guides/source/i18n.md

@@ -915,17 +915,7 @@ class User < ApplicationRecord
 end
 ```
 
-The key for the error message in this case is `:blank`. Active Record will look up this key in the namespaces:
-
-```
-activerecord.errors.models.[model_name].attributes.[attribute_name]
-activerecord.errors.models.[model_name]
-activerecord.errors.messages
-errors.attributes.[attribute_name]
-errors.messages
-```
-
-Thus, in our example it will try the following keys in this order and return the first result:
+The key for the error message in this case is `:blank`. Thus, in our example it will try the following keys in this order and return the first result:
 
 ```
 activerecord.errors.models.user.attributes.name.blank
@@ -935,6 +925,17 @@ errors.attributes.name.blank
 errors.messages.blank
 ```
 
+To explain it more abstractly, it returns the first key that matches in the order of the following list.
+
+```
+activerecord.errors.models.[model_name].attributes.[attribute_name].[key]
+activerecord.errors.models.[model_name].[key]
+activerecord.errors.messages.[key]
+errors.attributes.[attribute_name].[key]
+errors.messages.[key]
+```
+
+
 When your models are additionally using inheritance then the messages are looked up in the inheritance chain.
 
 For example, you might have an Admin model inheriting from User: