Add comment and improve variable name in roundrobin() (#4486)

Doc/library/itertools.rst

@@ -753,15 +753,16 @@ which incur interpreter overhead.
    def roundrobin(*iterables):
        "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
        # Recipe credited to George Sakkis
-       pending = len(iterables)
+       num_active = len(iterables)
        nexts = cycle(iter(it).__next__ for it in iterables)
-       while pending:
+       while num_active:
            try:
                for next in nexts:
                    yield next()
            except StopIteration:
-               pending -= 1
-               nexts = cycle(islice(nexts, pending))
+               # Remove the iterator we just exhausted from the cycle.
+               num_active -= 1
+               nexts = cycle(islice(nexts, num_active))
 
    def partition(pred, iterable):
        'Use a predicate to partition entries into false entries and true entries'