[MRG+1] Various docstring fixes for web docs

Various fixes to get proper formatting (bold, continuation lines & rendering of dicts basically) for the web API docs. Just looked at the `ensemble` and `linear_model` modules so far, but will try to find time to review others in a later PR. Motivated by noticing a lot of formatting problems on http://scikit-learn.org/stable/modules/generated/sklearn.linear_model.SGDClassifier.html