[css-images-3] Defined computed image, for #4042.

Author: tabatkins

css-images-3/Overview.bs

@@ -128,6 +128,10 @@ Image Values: the <<image>> type {#image-values}
 	and refresh the rendering as more data loads;
 	or it may wait until the entire image has loaded before using it.
 
+	A <dfn export>[=computed value|computed=] <<image>></dfn> value
+	is the [=specified value=]
+	with any <<url>>s, <<color>>s, and <<length>>s computed.
+
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