docs: add change to upgrade_emails

Author: kenjis

user_guide_src/source/installation/upgrade_emails.rst

@@ -14,6 +14,7 @@ Documentations
 What has been changed
 =====================
 - Only small things like the method names and the loading of the library have changed.
+- The behavior when using the SMTP protocol has been slightly changed. You may not be able to communicate with your SMTP server properly if you use the CI3 settings. See :ref:`email-ssl-tls-for-smtp` and :ref:`email-preferences`.
 
 Upgrade Guide
 =============

user_guide_src/source/libraries/email.rst

@@ -102,6 +102,8 @@ The settings used for the last successful send are available from the
 instance property ``$archive``. This is helpful for testing and debugging
 to determine that actual values at the time of the ``send()`` call.
 
+.. _email-preferences:
+
 Email Preferences
 =================