docs: ref: kernel: clarify data_item structure in message queues
docs: ref: kernel: clarify data_item structure in message queues

- use sizeof(struct data_item_type) instead of sizeof(data_item_type)
- use struct data_item_type instead of invalid reference data_item_t
- add aligned typedef struct data_item_type example implemenation
- add note about checking return value for error message

Fixes #19684

Signed-off-by: Jennifer Williams <7953176682adde4345743ea99dbf2682fb898b9d@intel.com>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr