T7253: Improve documentation about upgrades with recommendations

Improve the documentation about upgrades and rollbacks with information about how to handle settings.

content/designs/system-updates-and-rollback.md

@@ -740,6 +740,33 @@ image for decompression.
 The content of the update file is extracted into the temporary directory
 and the signature is checked for the extracted commit tree.
 
+### Configuration
+
+As described in [preferences and persistence]( {{< ref preferences-and-persistence.md >}} )
+there different types of settings which should be preserved across updates by
+either keeping them intact of updating them to match the new logic of the
+application.
+
+When using OSTree, ideally partitions are read-only, however, since system
+settings need a read-write support, some partitions are left read-write.
+These are the cases of `/etc` and `/var`. Also, since user data and
+settings also need to be preserved across updates, partition `/home`
+is also left as read-write.
+
+As explained, during an upgrade, a new commit is applied on the `OSTree` repo,
+which modifies the read-only partitions, but does not modify the read-write
+ones. To handle the upgrade of system settings stored in `/etc` a copy of its
+default values is kept in `/usr/etc` which is updated with the new commit.
+Thanks to this information `OSTree` can detect the files that have been changed
+and apply a 3-way merge, to update the `/etc`.
+
+This process allows to update settings to new defaults for files that were not
+modified and keep intact those that were.
+
+Applications are encouraged to handle settings adaptation to new version
+following the guidelines described in [user and user data management]( {{< ref "#user-and-user-data-management" >}} )
+and [preference and persistence]( {{< ref preferences-and-persistence.md >}} ).
+
 ### Error handling
 
 If for any reason the update process fails to complete, the update will