[css-borders-4] Clarify how border-shape renders box-shadow. (#13021)

Author: noamr

css-borders-4/Overview.bs

@@ -3079,7 +3079,8 @@ When an element's [=computed value=] of 'border-shape' is not <css>none</css>,
 its 'border-radius' is ignored, as if it was set to 0.
 'corner-shape' is implicitly ignored, as it can only work in tandem with 'border-radius'.
 
-A 'box-shadow' follows both the outer border path.
+An [=outer box-shadow=] follows the outside of the outer path, and an [=inner box-shadow=] follows the inside inner path.
+Both are rendered as a stroke, with a stroke width of <code>spread * 2</code>, clipped by the border shape.
 
 'border-shape' does not affect geometry or layout,
 which is still computed using the existing 'border-width' properties.