DOC: Updated third domain description
DOC: Updated third domain description

The description is of the cart-clearing task that is motivated by a
discussion with Brian Benoit in Oct 2014. It is placed in a new
subsection, named Summary, that is marked as non-normative.

fmrchallenge/fmrbenchmark,fmrchallenge/fmrbenchmark,fmrchallenge/fmrbenchmark