Clarify specification guidance in FFT functions

PR-URL: 	#720
Closes: #717
Closes: #718
Ref: #189
Co-authored-by: Leo Fang <[email protected]>
Co-authored-by: Athan Reines <[email protected]>
Reviewed-by: Leo Fang <[email protected]>
Reviewed-by: Athan Reines <[email protected]>
Reviewed-by: Ralf Gommers <[email protected]>

Author: 3 people