doc: settings: new API functions

Add references to the new API functions that were added
"csi_load_one" and "csi_get_val_len"

Signed-off-by: Riadh Ghaddab <rghaddab@baylibre.com>

Author: rghaddab

doc/services/storage/settings/index.rst

@@ -78,6 +78,14 @@ backend.
     This gets called when loading values from persistent storage using
     :c:func:`settings_load()`.
 
+**csi_load_one**
+    This gets called when loading only one item from persistent storage using
+    :c:func:`settings_load_one()`.
+
+**csi_get_val_len**
+    This gets called when getting a value's length from persistent storage using
+    :c:func:`settings_get_val_len()`.
+
 **csi_save**
     This gets called when saving a single setting to persistent storage using
     :c:func:`settings_save_one()`.
@@ -148,6 +156,14 @@ After all data is loaded, the ``h_commit`` handler is issued,
 signalling the application that the settings were successfully
 retrieved.
 
+Alternatively, a call to :c:func:`settings_load_one()` will load only one
+Settings entry and store it in the provided buffer.
+
+Optionally, to get only the value's length associated with the Settings entry,
+a call to :c:func:`settings_get_val_len()` can be performed.
+This is used for example by applications that allocates dynamically the data
+buffer and needs to get the data size before reading it by settings_load_one().
+
 Technically FCB and file backends may store some history of the entities.
 This means that the newest data entity is stored after any
 older existing data entities.