Tools: document GyroFFT FTN3 log message

Author: peterbarker

Tools/autotest/vehicle_test_suite.py

@@ -3092,7 +3092,6 @@ def LoggerDocumentation_greylist(self):
         '''returns a set of messages should should be documented but
         are currently known as undocumented'''
         return set([
-            "FTN3",  # gyrofft
             "SBRE",  # septentrio
 
             "SAF1",  # blimp-sim