Add comment for a pending issue.

src/component/graphic.js

@@ -401,6 +401,10 @@ echarts.extendComponentView({
                     height: parentEl.__ecGraphicHeight
                 };
 
+            // PENDING
+            // Currently, when `bounding: 'all'`, the union bounding rect of the group 
+            // does not include the rect of [0, 0, group.width, group.height], which
+            // is probably weird for users. Should we make a break change for it?
             layoutUtil.positionElement(
                 el, elOption, containerInfo, null,
                 {hv: elOption.hv, boundingMode: elOption.bounding}