Merge pull request #672 from yast/self_update_id

Extend current self_update_id doc.

doc/SELF_UPDATE.md

@@ -111,9 +111,19 @@ The URL of the update repository is evaluated in this order:
         required just like in manual installation.
    4. Default SUSE Customer Center API (`https://scc.suse.com/`).
 
-   Registration server is then asked for URL providing product on media.
-   For multi-product media it is possible to specify ID used instead of product
-   name in control.xml as globals->self_update_id.
+   The registration server is then asked for the list of update repositories.
+   In order to determine such list, the product `name`, `version` and
+   `architecture` are used
+
+   In case of a multi-product installation medium, as the installer is the same
+   for all the included products, the product `name` can be hard-coded in the
+   `control.xml` file with the `/globals/self_update_id` XML node.
+
+   ```xml
+   <globals>
+     <self_update_id>SLES</self_update_id>
+   </globals>
+   ```
 
 4. Hard-coded in the `control.xml` file on the installation medium (thus it
    depends on the base product):