frameline: add doc on econ roi

src/gateware/cxp_frame_pipeline.py

@@ -698,6 +698,7 @@ class ROI(Module):
             ),
         ]
 
+# NOTE: this "ECON" ROI use more Carry4 and introduce more tight setup/hold pins
 class Economical_ROI(Module):
     """
     ROI Engine. For each frame, accumulates pixels values within a