[css-images-3] Be more explicit about SVG image sources. w3c#5294

Author: fantasai

css-images-3/Overview.bs

@@ -1551,9 +1551,11 @@ Image Processing {#image-processing}
 	to be applied to the element’s images
 	before they are used in the document.
 	It applies to content images
-	(e.g. replaced elements and generated content),
-	as well as to decorative images applied via CSS
-	(such as 'background-image').
+	(e.g. replaced elements and generated content)
+	and image sources referenced by SVG elements
+	(such as <{feImage}>),
+	as well as to decorative images applied via CSS rules
+	(such as 'background-image' and other <<image>> properties).
 	It does not apply to the rendering of images outside the document,
 	e.g. favicons in the UA’s navigation toolbars or menus, etc.