[Docs] Explain school ID

docs/admin/20_configuration.rst

@@ -45,11 +45,29 @@ you have to set the following settings:
    host = "mysqlserver"
    port = 3306
 
+Preferences
+-----------
+
+The preferences for the import can be set from the menu under
+`Admin → Configuration → Untis`.
+
+Configure the school ID
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The only required preference is the Untis school ID. You need to
+provide this in all cases, even if your Untis database hosts only one
+school.
+
+.. warning::
+
+   If your Untis database hosts several schools, but you forget to
+   configure the school ID, data corruption may occur!
+
 Customise how data are imported
--------------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-The way data are imported can be configured from the menu under
-`Admin → Configuration → Untis`. The following preferences are available:
+The behaviour of the import can be customised in several ways. The
+following preferences are available:
 
 * **Update values of existing subjects:** This will update the values of
   already existing subjects if Untis has different data.