clarify overwrite_x in cupyx.scipy.fftpack
address and closes
@@ -25,3 +25,5 @@ Fast Fourier Transforms
Code compatibility features
---------------------------
The ``get_fft_plan`` function has no counterpart in ``scipy.fftpack``. It returns a cuFFT plan that can be passed to the FFT functions in this module (using the argument ``plan``) to accelarate the computation.
+
+Like in ``scipy.fftpack``, all FFT functions in this module have an optional argument ``overwrite_x`` (default is ``False``). Note that it has the same semantics as in ``scipy.fftpack``: when it is set to ``True``, the input array ``x`` *can* (not *will*) be destroyed and replaced by the output. Therefore, to guarantee an in-place FFT is performed, one should always do ``x = cupyx.scipy.fftpack.fft(x, ..., overwrite_x=True, ...)``.
