clarify levels in performance section, again

Author: j9ac9k

doc/source/api_reference/graphicsItems/imageitem.rst

@@ -58,8 +58,10 @@ The following guidance should be observed if performance is an important factor
 * For 1 or 3 channel data, use `uint8`, `uint16`, `float32`, or `float64` ``image``
   dtype.
 * For 4-channel data, use `uint8` or `uint16` with :python:`levels=None`.
-* ``levels`` should be set to (even if to ``None``) to avoid autoLevels sampling.
-* ``levels`` should be single channel or ``None``.
+* ``levels`` should be set to either to ``None`` or to single channel ``[min, max]``
+  
+  * Not setting ``levels`` will trigger autoLevels sampling
+
 * If using LUTs (lookup tables), ensure they have a dtype of `uint8` and have 256
   points or less, and that. That can be accomplished with calling: