doc: fix  documentation on Idle handles
:type loop: :py:class:`Loop`
:param loop: loop object where this handle runs (accessible through :py:attr:`Idle.loop`).
- ``Idle`` handles run when the event loop is *idle*, that is, there are no
- other events pending to be run. It is usually used to defer operations to
- be run at a later loop iteration.
+ ``Idle`` handles will run the given callback *once per loop iteration*, right
+ before the :py:class:`Prepare` handles.
+
+ .. note::
+ The notable difference with :py:class:`Prepare` handles is that
+ when there are active :py:class:`Idle` handles, the loop will perform
+ a zero timeout poll instead of blocking for I/O.
+
+ .. warning::
+ Despite the name, :py:class:`Idle` handles will get their callbacks
+ called on **every** loop iteration, not when the loop is actually
+ "idle".
.. py:method:: start(callback)
