[css-images-4] Add logical directions for linear-gradient()

Author: Psychpsyo

css-images-4/Overview.bs

@@ -1107,11 +1107,14 @@ Linear Gradients: the ''linear-gradient()'' notation {#linear-gradients}
 	when interpolating colors on the gradient line.
 	See [[css-color-4#interpolation]].
 
+	This level also adds the ability to define the gradient’s direction
+	relative to the current block and inline directions.
+
 	<pre class=prod>
 		<dfn>&lt;linear-gradient-syntax></dfn> =
 			[ [ <<angle>> | <<zero>> | to <<side-or-corner>> ] || <<color-interpolation-method>> ]? ,
 			<<color-stop-list>>
-		<dfn>&lt;side-or-corner></dfn> = [left | right] || [top | bottom]
+		<dfn>&lt;side-or-corner></dfn> = [ [left | right] || [top | bottom] ] | [ [inline-start | inline-end] || [block-start | block-end] ]
 	</pre>
 
 	<wpt>