settings: doc: Update the backend documentation

This commit updates the settings backend documentation
to clearly state that it cannot provide old entities
before the final one.

Signed-off-by: Radoslaw Koppel <radoslaw.koppel@nordicsemi.no>

doc/reference/storage/settings/settings.rst

@@ -94,13 +94,16 @@ Loading data from persisted storage
 
 A call to ``settings_load()`` uses an ``h_set`` implementation
 to load settings data from storage to volatile memory.
-For both FCB and filesystem back-end the most
-recent key values are guaranteed by traversing all stored content
-and (potentially) overwriting older key values with newer ones.
 After all data is loaded, the ``h_commit`` handler is issued,
 signalling the application that the settings were successfully
 retrieved.
 
+Technically FCB and filesystem backends may store some history of the entities.
+This means that the newest data entity is stored after any
+older existing data entities.
+Starting with Zephyr 2.1, the back-end must filter out all old entities and
+call the callback with only the newest entity.
+
 Storing data to persistent storage
 **********************************
 

include/settings/settings.h

@@ -390,6 +390,12 @@ struct settings_store_itf {
 	 * Parameters:
 	 *  - cs - Corresponding backend handler node,
 	 *  - arg - Structure that holds additional data for data loading.
+	 *
+	 * @note
+	 * Backend is expected not to provide duplicates of the entities.
+	 * It means that if the backend does not contain any functionality to
+	 * really delete old keys, it has to filter out old entities and call
+	 * load callback only on the final entity.
 	 */
 
 	int (*csi_save_start)(struct settings_store *cs);