#802 improve definition of "Non negative matrix factorization"

[jzheng]

The current definition of "Non negative matrix factorization":
“Non negative matrix factorization is a data transformation in which factorises a matrix and which forces that all elements must be equal to or greater than zero.”

Suggest change to
“A data transformation in which a matrix is factorized into (usually) two matrices with the property that all three matrices have no negative elements.”

Definition source:
https://en.wikipedia.org/wiki/Non-negative_matrix_factorization