missing normalisation term in ifft doc
doc: fix and clarify documentation for the `parse` function.
Merge pull request #6088 from simonbyrne/fftdoc

missing normalisation term in ifft doc
Hopefully improved clarity of FFT documentation. Ref: #6088

- Use IDFT formula in documentation of ifft, instead of BDFT
- Typeset FFT, IDFT and BDFT fomulae in math blocks instead of inline