[css-values] Specify that direction <angle>s are always bearing angles in CSS.

Author: tabatkins

css-values/Overview.bs

@@ -929,6 +929,16 @@ Angle Units: the <<angle>> type and ''deg'', ''grad'', ''rad'', ''turn'' units</
 	For example, a right angle is ''90deg'' or ''100grad'' or ''0.25turn'' or
 	approximately ''1.57rad''.
 
+	When an angle denotes a direction,
+	it must always be interpreted as a <dfn export>bearing angle</dfn>,
+	where 0deg is "up" or "north" on the screen,
+	and larger angles are more clockwise
+	(so 90deg is "right" or "east").
+
+	For example, in the ''linear-gradient()'' function,
+	the <<angle>> that determines the direction of the gradient
+	is interpreted as a bearing angle.
+
 <h3 id="time">
 Duration Units: the <<time>> type and ''s'', ''ms'' units</h3>