Add documentation about irFFTs and odd vs even sized output

[egpbos]

Real FFTs on arrays where the last number in the shape is an odd number require special care. In particular: an inverse rFFT cannot automatically determine whether the output should be an odd or even sized (real) array, so the user has to supply this information. I added a flag for this in the relevant functions, but this behavior is undocumented. Add to docs.