doc: storage: add flashdisk to disk documentation

Include zephyr,flash-disk devicetree sample in disk documentation.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>

Author: tmon-nordic

doc/services/storage/disk/access.rst

@@ -65,6 +65,26 @@ To read and write files and directories, see the :ref:`file_system_api` in
 :zephyr_file:`include/zephyr/fs/fs.h` such as :c:func:`fs_open()`,
 :c:func:`fs_read()`, and :c:func:`fs_write()`.
 
+Emulated block device on flash partition support
+************************************************
+
+Zephyr flashdisk driver makes it possible to use flash memory partition as
+a block device. The flashdisk instances are defined in devicetree:
+
+.. code-block:: devicetree
+
+    / {
+        msc_disk0 {
+            compatible = "zephyr,flash-disk";
+            partition = <&storage_partition>;
+            disk-name = "NAND";
+            cache-size = <4096>;
+        };
+    };
+
+The cache size specified in :dtcompatible:`zephyr,flash-disk` node should be
+equal to backing partition minimum erasable block size.
+
 Disk Access API Configuration Options
 *************************************