Document destructive inverse real FFTs

[egpbos]

The default behavior for inverse real FFT functions (c2r) in FFTW is to destroy the input arrays during the calculation. This is probably very unexpected for most users.

We should add this to the documentation, probably multiple times (in the general howtos, and in the function docs). Maybe we should even print run-time warnings.

[egpbos]

Todo: add notes to function documentation.