[css-images-4] Copy over definition of computed <image>. #4042

Author: fantasai

css-images-4/Overview.bs

@@ -51,7 +51,7 @@ Introduction {#intro}
 2D Image Values: the <<image>> type {#image-values}
 ===================================================
 
-The <<image>> value type denotes a 2D image. It can be a
+	The <<image>> value type denotes a 2D image. It can be a
 	<a section href="#url-notation">url reference</a>,
 	<a section href="#image-notation">image notation</a>,
 	or <a section href="#gradients">gradient notation</a>.
@@ -90,6 +90,11 @@ The <<image>> value type denotes a 2D image. It can be a
 	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.
+
+
 Image File Formats {#image-file-formats}
 ----------------------------------------