documentation: add TeX formulas to recommender API doc

[zenogantner]

As suggested by Yong Zheng:
https://groups.google.com/forum/?fromgroups=#!topic/mymedialite/_0fzNa0CGrg

This is done for UserItemBaseline, but it would be good to do this as well for all the matrix factorization methods, plus for all correlation/similarity measures.