Specify output shapes for fft functions

[asmeurer]

The fft functions do not specify what the output axis size should be. This is not necessarily always the same as the input axis size (or n).

For example, NumPy's ihfft uses an output shape of n//2 + 1 (https://numpy.org/doc/stable/reference/generated/numpy.fft.ihfft.html). This size is not mentioned anywhere in the spec.

[leofang]

Yes, I think it'd be nice to mention this for rfft, rfftn, and ihfft. Usually, the n or s input argument defines the length of the output along the transformed axis (and other axes have the same lengths), but for the 3 APIs doing R2C transform, n/s is for the input not output.