Remove and flip `index: true` for `references` in the doc [ci skip]

Follow up #32146.

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -348,8 +348,8 @@ def [](name)
       #
       #   create_table :taggings do |t|
       #     t.references :tag, index: { name: 'index_taggings_on_tag_id' }
-      #     t.references :tagger, polymorphic: true, index: true
-      #     t.references :taggable, polymorphic: { default: 'Photo' }
+      #     t.references :tagger, polymorphic: true
+      #     t.references :taggable, polymorphic: { default: 'Photo' }, index: false
       #   end
       def column(name, type, options = {})
         name = name.to_s

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -846,17 +846,17 @@ def index_name_exists?(table_name, index_name)
       # [<tt>:null</tt>]
       #   Whether the column allows nulls. Defaults to true.
       #
-      # ====== Create a user_id bigint column
+      # ====== Create a user_id bigint column without a index
       #
-      #   add_reference(:products, :user)
+      #   add_reference(:products, :user, index: false)
       #
       # ====== Create a user_id string column
       #
       #   add_reference(:products, :user, type: :string)
       #
-      # ====== Create supplier_id, supplier_type columns and appropriate index
+      # ====== Create supplier_id, supplier_type columns
       #
-      #   add_reference(:products, :supplier, polymorphic: true, index: true)
+      #   add_reference(:products, :supplier, polymorphic: true)
       #
       # ====== Create a supplier_id column with a unique index
       #
@@ -884,7 +884,7 @@ def add_reference(table_name, ref_name, **options)
       #
       # ====== Remove the reference
       #
-      #   remove_reference(:products, :user, index: true)
+      #   remove_reference(:products, :user, index: false)
       #
       # ====== Remove polymorphic reference
       #
@@ -892,7 +892,7 @@ def add_reference(table_name, ref_name, **options)
       #
       # ====== Remove the reference with a foreign key
       #
-      #   remove_reference(:products, :user, index: true, foreign_key: true)
+      #   remove_reference(:products, :user, foreign_key: true)
       #
       def remove_reference(table_name, ref_name, foreign_key: false, polymorphic: false, **options)
         if foreign_key

guides/source/association_basics.md

@@ -487,7 +487,7 @@ class CreatePictures < ActiveRecord::Migration[5.0]
   def change
     create_table :pictures do |t|
       t.string :name
-      t.references :imageable, polymorphic: true, index: true
+      t.references :imageable, polymorphic: true
       t.timestamps
     end
   end