mcm: comment on manual multiplication

Author: Spaqin

misoc/cores/duc.py

@@ -212,6 +212,9 @@ def __init__(self, width, constants):
         assert range(n) == constants
 
         ctx = self.sync if n > 8 else self.comb
+
+        # manually generated multiplication for small numbers,
+        # if you use "x*y" Vivado will use a DSP48E1 instead
         if n > 0:
             ctx += o[0].eq(0)
         if n > 1: