doc: migration-guide: 3.6: Add note on MCUmgr CRC change

Adds a note that an additional Kconfig needs to be selected if a
serial MCUmgr transport is used

(cherry picked from commit 3ce916d)

Original-Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
GitOrigin-RevId: 3ce916d
Change-Id: I2d999b9a03d9b335da4395aeef908f99b13e409b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/4999938
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>
Tested-by: Fabio Baltieri <fabiobaltieri@google.com>

doc/releases/migration-guide-3.6.rst

@@ -45,5 +45,9 @@ Networking
 Other Subsystems
 ================
 
+* MCUmgr applications that make use of serial transports (shell or UART) must now select
+  :kconfig:option:`CONFIG_CRC`, this was previously erroneously selected if MCUmgr was enabled,
+  when for non-serial transports it was not needed.
+
 Recommended Changes
 *******************