Merge pull request rubocop#923 from fatkodima/docs-reversible_migration

Improve examples for `ReversibleMigration` cop

Author: koic

lib/rubocop/cop/rails/reversible_migration.rb

@@ -16,23 +16,15 @@ module Rails
       #
       #   # good
       #   def change
-      #     create_table :users do |t|
-      #       t.string :name
+      #     change_table :users do |t|
+      #       t.remove :name, :string
       #     end
       #   end
       #
       #   # good
       #   def change
-      #     reversible do |dir|
-      #       change_table :users do |t|
-      #         dir.up do
-      #           t.column :name, :string
-      #         end
-      #
-      #         dir.down do
-      #           t.remove :name
-      #         end
-      #       end
+      #     create_table :users do |t|
+      #       t.string :name
       #     end
       #   end
       #
@@ -114,21 +106,6 @@ module Rails
       #     end
       #   end
       #
-      #   # good
-      #   def change
-      #     reversible do |dir|
-      #       change_table :users do |t|
-      #         dir.up do
-      #           t.change :price, :string
-      #         end
-      #
-      #         dir.down do
-      #           t.change :price, :integer
-      #         end
-      #       end
-      #     end
-      #   end
-      #
       # @example
       #   # remove_columns
       #