tests: smp: improving the description for SMP test cases
tests: smp: improving the description for SMP test cases

Add more detail description for test case of arch_curr_cpu() and
arch_sched_ipi(). This is in order to make the purpose and process of
the test cases more clear.

Signed-off-by: Enjia Mai <233e651694b7770d0f980a273a8e4a6f7e2a84d4@intel.com>

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