document new default hwm value

docs/source/parallel/parallel_task.txt

@@ -415,11 +415,11 b' assigned to an engine at a given time. This limit is set with the'
 .. sourcecode:: python
 
     # the most common choices are:
-    c.TaskSheduler.hwm = 0 # (minimal latency, default)
+    c.TaskSheduler.hwm = 0 # (minimal latency, default in IPython ≤ 0.12)
     # or
-    c.TaskScheduler.hwm = 1 # (most-informed balancing)
+    c.TaskScheduler.hwm = 1 # (most-informed balancing, default in > 0.12)
 
-The default is 0, or no-limit. That is, there is no limit to the number of
+In IPython ≤ 0.12,the default is 0, or no-limit. That is, there is no limit to the number of
 tasks that can be outstanding on a given engine. This greatly benefits the
 latency of execution, because network traffic can be hidden behind computation.
 However, this means that workload is assigned without knowledge of how long
@@ -429,6 +429,11 b' effect by setting hwm to a positive integer, 1 being maximum load-balancing (a'
 task will never be waiting if there is an idle engine), and any larger number
 being a compromise between load-balance and latency-hiding.
 
+In practice, some users have been confused by having this optimization on by
+default, and the default value has been changed to 1. This can be slower,
+but has more obvious behavior and won't result in assigning too many tasks to
+some engines in heterogeneous cases.
+
 
 Pure ZMQ Scheduler
 ------------------