Update NXdetector.nxdl.xml

base_classes/NXdetector.nxdl.xml

@@ -434,7 +434,8 @@
       Most often there is a single real time value that is constant across
       an entire image frame.  In such cases, only a 1-D array is needed.
       But there are detectors in which the real time
-      changes per pixel. In that case, more than one dimension is needed.
+      changes per pixel. In that case, more than one dimension is needed. Therefore
+      the rank of this field should be less than or equal to (detector rank + 1).
     </doc>
     <dimensions rank="3">
       <dim index="1" value="np" />