docs: Fix mailbox k_mbox_msg.tx_block documentation
docs: Fix mailbox k_mbox_msg.tx_block documentation

- Replace old tx_block.pool_id to tx_block.data;
- Fix code examples.

Signed-off-by: Rodrigo Peixoto <3b2e0cd8e32147e0545aeecd8920e147dee7f189@ayna.tech>
Signed-off-by: Rodrigo Peixoto <3b2e0cd8e32147e0545aeecd8920e147dee7f189@gmail.com>
Signed-off-by: Anas Nashif <0d9952ec84ac43c159f6b7e7ed99a9080c00dd6e@intel.com>

punitvara/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,ldts/zephyr,Vudentz/zephyr,kraj/zephyr,ldts/zephyr,ldts/zephyr,kraj/zephyr,galak/zephyr,finikorg/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr,kraj/zephyr,kraj/zephyr,ldts/zephyr