Clarify map() behavior.

Doc/library/functions.rst

@@ -640,7 +640,9 @@ available.  They are listed here in alphabetical order.
    Return an iterator that applies *function* to every item of *iterable*,
    yielding the results.  If additional *iterable* arguments are passed,
    *function* must take that many arguments and is applied to the items from all
-   iterables in parallel.
+   iterables in parallel.  With multiple iterables, the iterator stops when the
+   shortest iterable is exhausted.
+
 
 .. function:: max(iterable[, args...], *[, key])