[nrf fromtree] doc: storage: nvs: Tell about optional data CRC

The CONFIG_NVS_DATA_CRC configuration item allows to enable a CRC-32
on the data part of the NVS items.

Signed-off-by: Adrien Ricciardi <[email protected]>
(cherry picked from commit 61474ac)
Signed-off-by: Andrzej Głąbek <[email protected]>

Author: RICCIARDI-Adrien

doc/services/storage/nvs/nvs.rst

@@ -19,11 +19,16 @@ combination of these.
 Each element is stored in flash as metadata (8 byte) and data. The metadata is
 written in a table starting from the end of a nvs sector, the data is
 written one after the other from the start of the sector. The metadata consists
-of: id, data offset in sector, data length, part (unused), and a CRC. The CRC is
+of: id, data offset in sector, data length, part (unused), and a CRC. This CRC is
 only calculated over the metadata and only ensures that a write has been
-completed. The actual data of the element is not protected by a CRC. It is
-encouraged to include a CRC as part of the data and to take appropriate
-corrective actions when the data CRC does not match its expected value.
+completed. The actual data of the element can be protected by a different (and optional)
+CRC-32. Use the :kconfig:option:`CONFIG_NVS_DATA_CRC` configuration item to enable
+the data part CRC.
+**Note:** the data CRC is checked only when the whole data of the element is read.
+The data CRC is not checked for a partial read, as it is stored at the end of the
+element data area.
+**Note 2:** enabling the data CRC feature on a previously existing NVS content without
+data CRC will make all existing data invalid.
 
 A write of data to nvs always starts with writing the data, followed by a write
 of the metadata. Data that is written in flash without metadata is ignored