tests: drivers: can: api: add missing test case documentation
tests: drivers: can: api: add missing test case documentation

Add missing CAN controller API test case documentation.

Signed-off-by: Henrik Brix Andersen <24a3f13c22635bab658e39fc5f1e63c575328aae@vestas.com>

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