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.
DOC: document that first input to fftconvolve has to be larger than second input
Merge pull request #477 from rgommers/fftconvolve-valid-mode

Deprecate inconsistent behavior of fftconvolve valid mode in 0.12.x