upgrade: Add external config files section

Author: sirainen

docs/installation/upgrade/2.3-to-2.4.md

@@ -86,6 +86,8 @@ https://github.com/dovecot/tools/blob/main/dovecot-2.4.0-example-config.tar.gz
 
 <!-- @include: include/2.4-converted-settings.inc -->
 
+<!-- @include: include/2.4-external-configs.inc -->
+
 #### Default Settings
 
 | Setting | Old Default | New Default | Notes |

docs/installation/upgrade/include/2.4-external-configs.inc

@@ -0,0 +1,37 @@
+#### External Configuration Files
+
+v2.3 configured sql, ldap and some other features using `.conf.ext`
+external configuration files. These have been replaced by regular settings,
+which can be used in the same place where they used to be referred to before.
+For example:
+
+::: code-group
+```[dovecot-2.3.conf]
+passdb {
+  driver = mysql
+  args = /etc/dovecot/dovecot-sql.conf.ext
+}
+```
+
+```[dovecot-2.3-sql.conf.ext]
+connect = host=127.0.0.1 user=mysql_user pass=mysql_pass
+password_query = SELECT password FROM users WHERE user = '%u'
+```
+
+```[dovecot-2.4.conf.ext]
+# Use these mysql settings globally. These could be also inside passdb sql {}
+mysql 127.0.0.1 {
+  user = mysql_user
+  pass = mysql_pass
+}
+sql_driver = mysql
+
+passdb sql {
+  query = SELECT password FROM users WHERE user = '%{user}'
+}
+```
+:::
+
+ * [[link,auth_sql]]
+ * [[link,auth_ldap]]
+ * [[link,auth_oauth2]]