DisposeWhenIdle() never disposes

[bensmooth]

The memory in my app was growing by an alarming rate over time.
If you never call WaitForIdle(), nothing passed to DisposeWhenIdle() is disposed.

If we don't want to merge this pull request, we should at least update the documentation on DisposeWhenIdle(), so that it is clear that WaitForIdle() must be called for disposing to happen:
#350