4456-4457: add note on LEDs

4456-4457.tex

@@ -418,6 +418,12 @@
         \caption{Phase noise measurement}
     \end{figure}
 
+\section{Programmable LEDs}
+
+    4456 Mirny features several status LEDs, including a two per output channel. One per channel displays RF switch status.
+
+    The 4457 Almazny mezzanine features an additional row of LEDs, one per output channel, without a fixed purpose. The associated ARTIQ module allows programming these directly through the channel \texttt{set} method.
+
 \newpage
 \sysdescsection