[nrf fromlist] doc/releases/3.1: Add notes on MCUMGR to zcbor

... transition

The commit adds release notes on MCUMGR transition from TinyCBOR
to zcbor, and deprecation of several Kconfig options that are
no longer needed.

Upstream PR: zephyrproject-rtos/zephyr#45965

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

Author: de-nordic

doc/releases/release-notes-3.1.rst

@@ -208,6 +208,13 @@ Libraries / Subsystems
 
 * Management
 
+  * MCUMGR has been migrated from using TinyCBOR, for CBOR encoding, to zcbor.
+  * MCUMGR :kconfig:option:`CONFIG_FS_MGMT_UL_CHUNK_SIZE` and
+    :kconfig:option:`CONFIG_IMG_MGMT_UL_CHUNK_SIZE` have been deprecated as,
+    with the introduction of zcbor, it is no longer needed to use an intermediate
+    buffer to copy data out of CBOR encoded buffer. The file/image chunk size
+    is now limited by :kconfig:option:`CONFIG_MCUMGR_BUF_SIZE` minus all the
+    other command required variables.
   * Added support for MCUMGR Parameters command, which can be used to obtain
     MCUMGR parameters; :kconfig:option:`CONFIG_OS_MGMT_MCUMGR_PARAMS` enables
     the command.