improve LsiModel (SVD) documentation

[piskvorky]

Extend the theoretical part of LsiModel documentation:

• how do multipass/singlepass SVD relate and how can users turn them on/off
• describe speed/accuracy trade-offs of multipass vs. singlepass
• where do users find topic "details" (U, S matrices for LSI, lambda and gamma for LDA...)
• but don't confuse newbie users! find a proper place for this "advanced" info

http://groups.google.com/group/gensim/browse_thread/thread/4b605b72f8062770#

[piskvorky]

Re. online updates, http://groups.google.com/group/gensim/browse_thread/thread/3136fefaf859ddae :

More concretely: a practical amount of documents to add to the model at a time is at least "requested dimensionality (=num_topics)" + "internal oversampling (=100)".