DOC: sparse: don't recommend lil_matrix

Constructing a lil_matrix can easily take quadratic time, while a dok_matrix should always take linear time. Given that sparse matrices are intended to handle very large datasets, recommending the quadratic time algorithm is a bad idea. I've seen this go wrong on several occasions and it comes up on [StackOverflow](http://stackoverflow.com/q/18737657/166749) every now and then.

A voice at the back of my head says `dok_matrix` should be rewritten in C++ for better performance and more operations, but I'll leave that for some other time.