pyfastservo: add note for dac output override

fast-servo/pyfastservo/dac.py

@@ -108,6 +108,9 @@ def set_dac_output(value):
     low_word = value & 0xFF
     high_word = (value >> 8) & 0x3F
 
+    # Note: DAC HIGH word and LOW word output are not updated 
+    #   at the same time. On scope, you will see more than one step
+    #   of value changed.
     write_to_memory(CH0_HIGH_WORD_ADDR, high_word)
     write_to_memory(CH0_LOW_WORD_ADDR, low_word)
     write_to_memory(CH1_HIGH_WORD_ADDR, high_word)