Cleanup: Add comment explaining DPI influence on RNA pixel-properties

Good to be explicit about the fact that we may still use the pixel
property sub-type when DPI will be applied.
See comments in https://developer.blender.org/D7077.

source/blender/makesrna/RNA_types.h

@@ -121,6 +121,8 @@ typedef enum PropertySubType {
   PROP_PASSWORD = 6,
 
   /* numbers */
+  /** A dimension in pixel units, possibly before DPI scaling (so value may not be the final pixel
+   * value but the one to apply DPI scale to). */
   PROP_PIXEL = 12,
   PROP_UNSIGNED = 13,
   PROP_PERCENTAGE = 14,