Update MessageEncryptor guide to show that an exception is raised

swap order

activesupport/lib/active_support/message_encryptor.rb

@@ -22,6 +22,11 @@ module ActiveSupport
   #   crypt = ActiveSupport::MessageEncryptor.new(key)                            # => #<ActiveSupport::MessageEncryptor ...>
   #   encrypted_data = crypt.encrypt_and_sign('my secret data')                   # => "NlFBTTMwOUV5UlA1QlNEN2xkY2d6eThYWWh..."
   #   crypt.decrypt_and_verify(encrypted_data)                                    # => "my secret data"
+  # The +decrypt_and_verify+ method will raise an
+  # <tt>ActiveSupport::MessageEncryptor::InvalidMessage</tt> exception if the data
+  # provided cannot be decrypted or verified.
+  #
+  #   crypt.decrypt_and_verify('not encrypted data') # => ActiveSupport::MessageEncryptor::InvalidMessage
   #
   # === Confining messages to a specific purpose
   #