Outdated docstring for AlternatingLeastSquares.fit

[madcat1991]

Hi @benfred ,

It seems that the documentation for AlternatingLeastSquares.fit is slightly outdated. In particular the part related to the item_users parameter:

The negative items are implicitly defined: This code assumes that non-zero items in the item_users matrix means that the user liked the item. The negatives are left unset in this sparse matrix: the library will assume that means Piu = 0 and Ciu = 1 for all these items.

According to the issue #119 and this diff it is possible now to provide negative Ciu values to take into account explicit negative feedback.

If so, then the documentation should be updated.

[benfred]

Thanks! updated the docstring here: ee95496