Document that hook_mail_alter passes a langcode instead of a language object

Problem/Motivation

In D8 we adopted the use of langcodes to indicate the language of an email (ref drupal_mail(), hook_mail() and hook_mail_alter() now take langcode instead of language objects), but we never got round to updating the documentation of hook_mail_alter() to reflect this change. It still mentions the use of language objects.

Since the mail system is being deprecated in #1803948: [META] Adopt the symfony mailer component this is a minor issue, but it is still be a good idea to update the documentation because the current mail system will be around until Drupal 12 is release (or possibly longer).