DOC: add note about using fftshift, change Nyquist sign

1. Added a note about using fftshift to re-center the frequencies
2. Changed the description so that the Nyquist frequency is considered negative, since this is how fftfreq and fftshift do it:

        In : fftshift(fftfreq(8, 1/8))
        Out: array([-4., -3., -2., -1.,  0.,  1.,  2.,  3.])
3. backticks around variables, whitespace changes

For #2, the description is different when n is odd, though:

    In [31]: fftfreq(8, 1/8)
    Out[31]: array([ 0.,  1.,  2.,  3., -4., -3., -2., -1.])
    
    In [32]: fftfreq(7, 1/7)
    Out[32]: array([ 0.,  1.,  2.,  3., -3., -2., -1.])

So maybe the description should say 

> A[1:(n+1)//2] contains the  positive-frequency terms, and 
> A[(n+1)//2:] contains the negative-frequency terms

but that's more complicated