lib/DelayLine: add a bit more documentation

lib/DelayLine.bsv

@@ -77,6 +77,11 @@ module mkDelayLine(Integer delay_cycles, DelayLine#(a) ifc)
          delay = cons(r, delay);
       end
 
+      // pump_line ingests a new value into the first register in the
+      // line, and shifts older values one register down. The
+      // registers always shift forward even when nobody's writing, in
+      // which case that slot in the pipeline is occupied by an empty
+      // Maybe.
       (* no_implicit_conditions, fire_when_enabled *)
       rule pump_line;
          delay[0] <= inputVal.wget();