add documentation for overwriting behaviour of in-place application of real-input fft plans #88
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As noted in JuliaMath/FFTW.jl#158, application of inverse plans of real-input ffts via
mul!orldiv!potentially overwrites the input array.This might be quite unexpected to many people (including me), especially since this is not the case for the complex-input versions.
This PR is an attempt at documenting this behaviour.