Added information about how to compute stripes()

Author: SebastianZ

css-images-4/Overview.bs

@@ -1934,6 +1934,9 @@ Color Stop “Fixup”</h4>
 	is smaller than the total thickness of the stripes, the stripes are clipped
 	starting from the last defined color.
 
+	The computed value of this function is the [=specified value=] with all
+	<<color>>s computed '1fr' values removed.
+
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