Deprecate inconsistent behavior of fftconvolve valid mode in 0.12.x

   DEP: deprecate behavior of valid mode in fftconvolve where inputs were switched
    
    Note that this behavior was inconsistent with that of convolve.  In master the
    same function is now used for all valid modes in signaltools, so they are
    consistent.  In 66fe7c8929 @jseabold noted that outright making the change
    caused an issue in statsmodels, so deprecation in 0.12.x is needed.

The second commit (doc update) should be applied to master as well.