add docstring for cross_tab method
add docstring for cross_tab method

pombredanne/lsh-hdc,pombredanne/lsh-hdc,escherba/lsh-hdc,pombredanne/lsh-hdc,pombredanne/lsh-hdc,escherba/lsh-hdc,escherba/lsh-hdc,escherba/lsh-hdc,pombredanne/lsh-hdc,escherba/lsh-hdc