improve the interpolate docs

This is a follow-up to gh-3990:

* tweak the docstring example of `interp1d` so that the plot renders
* remove duplicated entries from the module documentation
* expand the docstrings of the *UnivariateSpline methods, fix small errors, document small quirks.

This is by no means a release blocker, but it could be nice to have  in the 0.15 branch.

[ci skip]
Minor reST issue: there need to be double backticks around ``2*k``. Otherwise the ``*`` is interpreted as markup.