Add a simple example of using the teardown phase

[DhairyaLGandhi]

Currently, the setup phase is documented properly, but a similar example of the teardown would be well appreciated as that quickly got lost when trying to work on some benchmarks I was working on recently. It wasn't obvious that my benchmark (specifically the teardown) was set up properly, and there wasn't a simple one-or-two liner in the docs that I could use to verify what I was doing was in fact how it is supposed to be.

[bjarthur]

further, the docstring for @benchmark doesn't even mention teardown