[css-backgrounds-3] Slight clarifying wording tweaks to shadow radius rules.

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -2917,19 +2917,20 @@ Shadow Shape, Spread, and Knockout</h4>
 
   <p>To preserve the box's shape when spread is applied,
   the corner radii of the shadow are also increased (decreased, for inner shadows)
-  from the border-box radii by adding (subtracting)
+  from the border-box (padding-box) radii by adding (subtracting)
   the <i>spread distance</i> (and flooring at zero).
   However, in order to create a sharper corner when the border radius is small
   (and thus ensure continuity between round and sharp corners),
   when the <i>border radius</i> is less than the <i>spread distance</i>
   (or in the case of an inner shadow,
   less than the absolute value of a negative <i>spread distance</i>),
   the <i>spread distance</i>
-  is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
+  is first multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
   where <var>r</var> is the ratio of the border radius to the <i>spread distance</i>,
   in calculating the corner radii of the spread shadow shape.
   For example, if the border radius is 10px and the <i>spread distance</i> is 20px (<var>r</var> = .5),
-  the corner radius of the shadow shape will be 10px + 20px × (1 + (.5 - 1)<sup>3</sup>) = 27.5px.
+  the corner radius of the shadow shape will be 10px + 20px × (1 + (.5 - 1)<sup>3</sup>) = 27.5px
+  rather than 30px.
   This adjustment is applied independently to the radii in each dimension.
 
   <p>The 'border-image' does not affect the shape of the box-shadow.