DOC: remove docstring from UnivariateSpline.__init__

same for InterpolatedUnivariateSpline and LSQUnivariateSpline.
These simply duplicated the class docstrings.
DOC: improve the docstrings of *UnivariateSpline classes

Tweak examples to make plots actually render.
DOC: add a nan masking example to the UnivariateSpline docstring

example is by @juliantaylor,
https://github.com/scipy/scipy/issues/3531#issuecomment-40233093
DOC: mention Schoenberg-Whitney conditions in the LSQ*Spline docstring
DOC: use spl(x) in the *Spline docstrings

to avoid confusion between s(x) being the spline function
and s being the (scalar) smoothing parameter
Merge pull request #3990 from ev-br/unv_spl_docstr

improve *UnivariateSpline docs