Add StopIteration hint to docs

docs/adding_tracks.rst

@@ -804,7 +804,7 @@ In ``register`` you bind the name in the track specification to your parameter s
 * The constructor needs to have the signature ``__init__(self, track, params, **kwargs)``.
 * ``partition(self, partition_index, total_partitions)`` is called by Rally to "assign" the parameter source across multiple clients. Typically you can just return ``self``. If each client needs to act differently then you can provide different parameter source instances here as well.
 * ``params(self)``: This method returns a dictionary with all parameters that the corresponding "runner" expects. This method will be invoked once for every iteration during the race. In the example, we parameterize the query by randomly selecting a profession from a list.
-* ``infinite``: This property helps Rally to determine whether to let the parameter source determine when a task should be finished (usually when ``infinite`` is ``False``) or whether the task properties (e.g. ``iterations`` or ``time-period``) determine when a task should be finished.
+* ``infinite``: This property helps Rally to determine whether to let the parameter source determine when a task should be finished (when ``infinite`` is ``False``) or whether the task properties (e.g. ``iterations`` or ``time-period``) determine when a task should be finished. In the former case, the parameter source needs to raise ``StopIteration`` to indicate when it is finished.
 
 For cases, where you want to provide a progress indication (this is typically the case when ``infinite`` is ``False``), you can implement a property ``percent_completed`` which returns a floating point value between ``0.0`` and ``1.0``. Rally will query this value before each call to ``params()`` and uses it to indicate progress. However: