Clarified spectral inversion.

libraries/dsp/si/src/python/si.py

@@ -36,6 +36,8 @@
 #     1st Nyquist zone: f =   0 - 100 MHz
 #     2nd Nyquist zone: f = 100 - 200 MHz
 #     3rd Nyquist zone: f = 200 - 300 MHz
+#   Spectral inversion flips the frequency bands (Nyquist zones), it does not
+#   shift frequencies.
 
 import argparse
 import textwrap