DOC: Savitzky-Golay filter version added

Closes https://github.com/scipy/scipy/issues/3876.
This should be a complete sentence, and since there are only two new public functions, they might as well be named instead of saying "...and other functions related to...".  Perhaps something like

    The Savitzky-Golay filter was added with the new functions
    `scipy.signal.savgol_filter` and `scipy.signal.savgol_coeffs`.