doc: nrf-bm: libraries: bm_zms: explain clearing

Adds section that explains how to handle the storage system
clear.

Signed-off-by: Mirko Covizzi <mirko.covizzi@nordicsemi.no>

Author: MirkoCovizzi

doc/nrf-bm/libraries/bm_zms.rst

@@ -138,6 +138,14 @@ After that, it checks that the sector after this one is empty, or it will erase
 If this initialization is successful, the library sets the flag ``bm_zms_fs.init_flags.initialized`` to true.
 For asynchronous storage backends, you must wait for the initialization to finish before triggering a write or read operation.
 
+Clearing the storage system
+===========================
+
+Call the :c:func:`bm_zms_clear` function to clear the storage.
+
+When this uninitialization is successful, the library sets the flag :c:member:`bm_zms_fs.init_flags.initialized` to false.
+For asynchronous storage backends, you must wait for the uninitialization to finish before reinitializing the storage system.
+
 BM_ZMS ID/data write
 ====================