Merge pull request w3c#320 from w3c/fix-319

[geometry-1] Clarify what happens on missing scaleY for scale() funct…

Author: chrishtr

geometry/Overview.bs

@@ -1317,6 +1317,8 @@ export>is 2D</dfn>. This flag indicates that:
  <var>originY</var>, <var>originZ</var>)</dfn>
  <dd>
   <ol>
+   <li><p>If <var>scaleY</var> is missing, set <var>scaleY</var> to the value of <var>scaleX</var>.
+
    <li><p>Let <var>result</var> be the resulting matrix initialized to the values of the current
    matrix.