twister: add comment about stats logic

class ExecutionCounter(object):

    def __init__(self, total=0):

        '''

        Most of the stats are at test instance level

        Except that "_cases" and "_skipped_cases" are for cases of ALL test instances

        total complete = done + skipped_filter

        total = yaml test scenarios * applicable platforms

        complete perctenage = (done + skipped_filter) / total

        pass rate = passed / (total - skipped_configs)

        '''

        # instances that go through the pipeline

        # updated by report_out()

        self._done = Value('i', 0)

        # instances that actually executed and passed

        # updated by report_out()

        self._passed = Value('i', 0)

        # static filter + runtime filter + build skipped

        # updated by update_counting_before_pipeline() and report_out()

        self._skipped_configs = Value('i', 0)

        # cmake filter + build skipped

        # updated by report_out()

        self._skipped_runtime = Value('i', 0)

        # staic filtered at yaml parsing time

        # updated by update_counting_before_pipeline()

        self._skipped_filter = Value('i', 0)

        # updated by update_counting_before_pipeline() and report_out()

        self._skipped_cases = Value('i', 0)

        # updated by report_out() in pipeline

        self._error = Value('i', 0)

        self._failed = Value('i', 0)

        # initialized to number of test instances

        self._total = Value('i', total)

        # updated in update_counting_after_pipeline()

        self._cases = Value('i', 0)

        self.lock = Lock()