improve *UnivariateSpline docs

* document @juliantailor's trick of masking nans (https://github.com/scipy/scipy/issues/3531#issuecomment-40233093)
* remove docstring from `__init__` methods, as they simply duplicate the class docstrings
* tweak docstring examples so that plots actually render

If #3531 is viewed as a documentation issue, this PR fixes it.

[ci skip]