DT8: clarify why "QUERY ACTUAL LEVEL" is needed

I was wondering why the code was sending a seemingly useless command,
but it turns out that this is explicitly mandated by a note in the
standard. Let's put that into a comment.

dali/gear/sequences.py

@@ -69,6 +69,7 @@ def QueryDT8ColourValue(
             "'query' must be a value from QueryColourValueDTR enumerator"
         )
 
+    # 62386-209, command 250, Note 2: start by sending "QUERY ACTUAL LEVEL"
     yield QueryActualLevel(address)
     yield DTR0(query.value)
     msb = yield QueryColourValue(address)