SGD documentation updates

This PR makes a few documentation changes to SGDClassifier and SGDRegressor:

* Documents all the loss functions added in #810.
* Fixes the description of `epsilon`, which was incorrect for the Huber loss.
* Fixes a minor mistake in the docstring of `_init_t`.

Also: I don't understand why, but when I run `make doc-noplot`, these changes don't show up in the generated html. Am I doing something wrong?