[css-transforms-2] Add note that computed value of translate has percentages (w3c#6146)

While the premise of w3c#2124 was that this was not already defined, I
believe this is already the straightforward result of the definition in
cssom-1.  Thus this adds a note rather than additional normative text.

Fixes w3c#2124.

Author: dbaron

css-transforms-2/Overview.bs

@@ -612,6 +612,11 @@ If three values are given,
 this specifies a 3d translation,
 equivalent to the ''translate3d()'' function.
 
+Note: The <a>resolved value</a> of the 'translate' property
+is the <a>computed value</a>,
+and thus {{Window/getComputedStyle()}}
+includes percentage values in its results.
+
 <pre class="propdef">
 Name: rotate
 Value: none | <<angle>> | [ x | y | z | <<number>>{3} ] && <<angle>>