DOC: Savitzky-Golay filter version added

Closes https://github.com/scipy/scipy/issues/3876.
In a discussion in another PR, I noted that the convention for `versionadded` had been to put it in the `Notes` section, and Ralf agreed.  I think that's a convention we should stick to.  (Someday I'll put together a PR with a more detailed style guide for scipy docstrings.)