[css-borders-4] Update box-shadow interaction with border-shape (#13730)

Author: danielsakhapov

css-borders-4/Overview.bs

@@ -3347,10 +3347,10 @@ Interaction with 'border-radius' and 'corner-shape'</h4>
 <h4 id="border-shape-shadow-interaction">
 Interaction with 'box-shadow'</h4>
 
-	An [=outer box-shadow=] follows the outside of the outer path,
-	and an [=inner box-shadow=] follows the inside of the inner path.
-	Both are rendered as a stroke, with a stroke width of <code>spread * 2</code>,
-	clipped by the border shape.
+	An [=outer box-shadow=] is cast as if the shape defined by the outer path were opaque.
+	An [=inner box-shadow=] is cast as if everything outside the shape defined by the inner path were opaque.
+	For both inner and outer shadows, the shadow shape is expanded or contracted by the spread distance,
+	blurred by the blur radius, and then clipped by the 'border-shape', adhering to the standard 'box-shadow' painting rules.
 
 	<div class="example">
 		<pre class=lang-css>