[css-images] Add a math hint for calculating the length of the gradient line.

Author: tabatkins

css-images/Overview.bs

@@ -1059,6 +1059,20 @@ The linear gradient syntax is:
 	would intersect the corner of the <a>gradient box</a> in the specified direction.
 	The starting point is determined identically, but in the opposite direction.
 
+	<div class='note'>
+		Given:
+
+		<ul>
+			<li><var>A</var> the angle (in any quadrant) defining the gradient line’s direction such that 0 degrees points upwards and positive angles represent clockwise rotation,
+			<li><var>W</var> the width of the gradient box,
+			<li><var>H</var> the height of the gradient box,
+		</ul>
+
+		The length of the gradient line (between the <a>starting point</a> and <a>ending point</a>) is:
+
+		<code>abs(<var>W</var> * sin(<var>A</var>)) + abs(<var>H</var> * cos(<var>A</var>))</code>
+	</div>
+
 	Note: It is expected that the next level of this module will provide the ability to define the gradient's direction relative to the current text direction and writing-mode.
 
 	<div class=example>