DOC: hfft/ihfft description conflicts with itself

Says Hermite spectrum when it means signal
DOC: Add examples, copy some text from rfft functions to clarify inputs and outputs
DOC: add note clarifying why ifftshift is needed
DOC: change "Hermite" to "Hermitian", "though" to "although"
DOC: Remove parentheses around axis
DOC: Unlink non-variable 'm', use monospace instead
Merge pull request #3647 from endolith/patch-1

DOC: hfft/ihfft description conflicts with itself