Warn when using static locking beatnote freqs

lisainstrument/instrument.py

@@ -540,7 +540,11 @@ class Instrument:
             fplan: `fplan` parameter, c.f. `__init__()`
         """
         if fplan == 'static':
-            logger.info("Using default set of locking beatnote frequencies")
+            logger.warning(
+                "Using default set of locking beatnote frequencies; this "
+                "might cause interferometric beatnote frequencies to fall "
+                "outside the requirement range of 5..25 MHz"
+            )
             self.fplan_file = None
             self.fplan = ForEachMOSA({
                 '12': 8E6, '23': 9E6, '31': 10E6,