Ensure type_for_attribute method docs are visible [ci-skip]

`ActiveModel::AttributeRegistration` is marked `:nodoc:`, so
`type_for_attribute` must be documented under `ActiveModel::Attributes`,
which includes `ActiveModel::AttributeRegistration`.

Also, RDoc does not correctly interpret `:method:` docs when they are
immediately followed by a visibility keyword.  Therefore, this commit
adds delimiter comments before the keywords, similar to
4726b1a.

activemodel/lib/active_model/attribute_registration.rb

@@ -40,11 +40,6 @@ def attribute_types # :nodoc:
         end
       end
 
-      # Returns the type of the specified attribute after applying any
-      # modifiers. This method is the only valid source of information for
-      # anything related to the types of a model's attributes. The return value
-      # of this method will implement the interface described by
-      # ActiveModel::Type::Value (though the object itself may not subclass it).
       def type_for_attribute(attribute_name, &block)
         attribute_name = resolve_attribute_name(attribute_name)
 

activemodel/lib/active_model/attributes.rb

@@ -75,6 +75,19 @@ def attribute_names
         attribute_types.keys
       end
 
+      ##
+      # :method: type_for_attribute
+      # :call-seq: type_for_attribute(attribute_name, &block)
+      #
+      # Returns the type of the specified attribute after applying any
+      # modifiers. This method is the only valid source of information for
+      # anything related to the types of a model's attributes. The return value
+      # of this method will implement the interface described by
+      # ActiveModel::Type::Value (though the object itself may not subclass it).
+      #--
+      # Implemented by ActiveModel::AttributeRegistration::ClassMethods#type_for_attribute.
+
+      ##
       private
         def define_method_attribute=(name, owner:)
           ActiveModel::AttributeMethods::AttrNames.define_attribute_accessor_method(

activerecord/lib/active_record/attributes.rb

@@ -253,11 +253,14 @@ def _default_attributes # :nodoc:
       # :method: type_for_attribute
       # :call-seq: type_for_attribute(attribute_name, &block)
       #
-      # See ActiveModel::AttributeRegistration::ClassMethods#type_for_attribute.
+      # See ActiveModel::Attributes::ClassMethods#type_for_attribute.
       #
       # This method will access the database and load the model's schema if
       # necessary.
+      #--
+      # Implemented by ActiveModel::AttributeRegistration::ClassMethods#type_for_attribute.
 
+      ##
       protected
         def reload_schema_from_cache(*)
           reset_default_attributes!