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 <jennifer.m.williams@intel.com>