Reference haxe.Timer.stamp from Date.getTime (#7518)

std/Date.hx

@@ -53,6 +53,9 @@ extern class Date
 	/**
 		Returns the timestamp (in milliseconds) of the date. It might
 		only have a per-second precision depending on the platforms.
+
+		For measuring time differences with millisecond accuracy on
+		all platforms, see `haxe.Timer.stamp`.
 	**/
 	function getTime() : Float;