Update computed value lines to reference computed <image>. w3c#4042

Author: fantasai

css-masking-1/Overview.bs

@@ -424,7 +424,7 @@ Initial: none
 Applies to: All elements. In SVG, it applies to <a>container elements</a> excluding the <a element>defs</a> element, all <a>graphics elements</a> and the <a element>use</a> element
 Inherited: no
 Percentages: n/a
-Computed value: as specified, but with URIs made absolute
+Computed value: the keyword ''mask-image/none'', a computed <<image>>, or a computed <<url>>
 Media: visual
 Animation type: discrete
 </pre>
@@ -954,7 +954,7 @@ Initial: none
 Applies to: All elements. In SVG, it applies to <a>container elements</a> excluding the <a element>defs</a> element, all <a>graphics elements</a> and the <a element>use</a> element
 Inherited: no
 Percentages: n/a
-Computed value: ''mask-border-source/none'' or the image with its URI made absolute
+Computed value: they keyword ''mask-border-source/none'' or the computed <<image>>
 Media: visual
 Animation type: discrete
 </pre>