sampling: update module docstring

Make sampling module docstring reflect reality.

Change-Id: I47d51c51619d7babf493e1f5c0c85458d8e19224
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33785
Reviewed-by: Dan Kenigsberg <danken@redhat.com>