DOC: Note FFT type promotion (numpy#14912)

* DOC: Note FFT type promotion

The NumPy FFT implementation will promote `float32` types to `float64` types. This may not be desired for some applications, but the SciPy implementation supports more data types.

Closes numpy#14892

Author: carterbox

numpy/fft/__init__.py

@@ -118,8 +118,16 @@
 It differs from the forward transform by the sign of the exponential
 argument and the default normalization by :math:`1/n`.
 
+Type Promotion
+--------------
+
+`numpy.fft` promotes ``float32`` and ``complex64`` arrays to ``float64`` and
+``complex128`` arrays respectively. For an FFT implementation that does not
+promote input arrays, see `scipy.fftpack`.
+
 Normalization
 -------------
+
 The default normalization has the direct transforms unscaled and the inverse
 transforms are scaled by :math:`1/n`. It is possible to obtain unitary
 transforms by setting the keyword argument ``norm`` to ``"ortho"`` (default is