Clarified overlay transparency instructions (#1619)

docs/config.rst

@@ -594,6 +594,12 @@ Rendering
     called "night".  You want to create a Biome Overlay to be displayed on top
     of the "day" render.  Your config file might look like this:
 
+    .. note::
+
+        When 'overlay' is used the ``imgformat`` must be set to a transparent image
+        format like ``"png"``. Otherwise the overlay is rendered without transparency
+        and the render underneath will not show.
+
     ::
 
         outputdir = "output_dir"