[css-transforms-2] Specify when 'translate' is 2d vs 3d.

Author: tabatkins

css-transforms-2/Overview.bs

@@ -557,6 +557,13 @@ each specifying a translation against one axis,
 in the order X, Y, then Z.
 Unspecified translations default to ''0px''.
 
+If only one or two values or given,
+this specifies a 2d translation,
+equivalent to the ''translate()'' function.
+If three values are given,
+this specifies a 3d translation,
+equivalent to the ''translate3d()'' function.
+
 <pre class="propdef">
 Name: rotate
 Value: none | [ x | y | z | <<number>>{3} ]? && <<angle>>