tweak comment

src/util/graphic.js

@@ -312,9 +312,13 @@ function singleEnterEmphasis(el) {
     // where properties of `emphasis` may not appear in `normal`. We previously use
     // module:echarts/util/model#defaultEmphasis to merge `normal` to `emphasis`.
     // But consider rich text and setOption in merge mode, it is impossible to cover
-    // all properties in merge. So we use merge mode when setting style here, where
-    // only properties that is not `null/undefined` can be set. The disadventage:
-    // null/undefined can not be used to remove style any more in `emphasis`.
+    // all properties in merge. So we use merge mode when setting style here. 
+    // But we choose the merge strategy that only properties that is not `null/undefined`.
+    // Because when making a textStyle (espacially rich text), it is not easy to distinguish 
+    // `hasOwnProperty` and `null/undefined` in code, so we trade them as the same for simplicity.
+    // But this strategy brings a trouble that `null/undefined` can not be used to remove 
+    // style any more in `emphasis`. Users can both set properties directly on normal and 
+    // emphasis to avoid this issue, or we might support `'none'` for this case if required.
     targetStyle.extendFrom(hoverStl);
 
     setDefaultHoverFillStroke(targetStyle, hoverStl, 'fill');
@@ -356,8 +360,6 @@ function singleEnterNormal(el) {
         var normalStl = el.__cachedNormalStl;
         if (normalStl) {
             rollbackDefaultTextStyle(style);
-            // Consider null/undefined value, should use
-            // `setStyle` but not `extendFrom(stl, true)`.
             el.setStyle(normalStl);
             applyDefaultTextStyle(style);
         }