twister: add comment about stats logic
twister: add comment about stats logic

Add comments and docstring to class  ExecutionCounter to explain
the twsiter stats logic.

Signed-off-by: Ming Shao <6bd49e8bc84587c4ae9697e13665e9c193a3e343@intel.com>

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