[css-backgrounds-3] Mention text as a thing that can be overlapped. w3c#2476

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -2889,7 +2889,8 @@ Layering, Layout, and Other Details</h4>
 
   <p>The shadow effects are applied front-to-back:
   the first shadow is on top and the others are layered behind.
-  Shadows do not influence layout and may overlap other boxes or their shadows.
+  Shadows do not influence layout and may overlap (or be overlapped by)
+  other boxes and text or their shadows.
   In terms of stacking contexts and the painting order,
   the <i>outer box-shadows</i> of an element are drawn immediately below the background of that element,
   and the <i>inner shadows</i> of an element are drawn immediately above the background of that element