Clarify fftshift for N is even and odd.

applications/lofar2/model/pfb_os/dsp_study_erko.txt

@@ -212,20 +212,24 @@
       -0.5, 0, 0.5 [fs]
   . N even, e.g. N = 4:
 
-         <---- N = 4 ----->
-         0         fs/2          fs
-         |         |             |
-     n = 0    1    2    3
-         0/4  1/4  2/4  3/4      4/4
-         DC   positive  negative
+         <------ N = 4 ------->
+         0           fs/2           ( fs  )
+         |             |            ( |   )
+     n = 0    1        2    3       ( |   )
+         0/4  1/4      2/4  3/4     ( 4/4 )
+         DC   positive |    negative
+                       |
+                       \--> fftshift([0, 1, 2, 3]) = [2, 3, 0, 1]
 
   . N odd, e.g. N = 5:
          <------- N = 5 -------->
-         0           fs/2          fs
-         |             |           |
-     n = 0    1    2   | 3    4    |
-         0/5  1/5  2/5 | 3/5  4/5  5/5
+         0           fs/2          ( fs  )
+         |             |           ( |   )
+     n = 0    1    2   | 3    4    ( |   )
+         0/5  1/5  2/5 | 3/5  4/5  ( 5/5 )
          DC   positive | negative
+                       |
+                       \-->fftshift([0, 1, 2, 3, 5]) = [3, 4, 0, 1, 2]
 
   . With K = N // 2:
     . N even : DC, K - 1 positive, fs/2, K - 1 negative frequencies