docs: update the description of barChart.bar.style.fillOpacity

- add `function` tag to show it could be set as function
- add an example

#3894

Author: shiquda

docs/assets/option/en/graphic/fill-style.md

@@ -10,9 +10,13 @@ Fill color. Supports setting to pure color in ways like `rgb(255,255,255)`, `rgb
 
 {{ use: graphic-gradient }}
 
-#${prefix} fillOpacity(number)
+#${prefix} fillOpacity(number|function)
 
-Fill opacity.
+Fill opacity. Supports dynamic setting using callback functions, with the callback function defined as follows:
+
+```ts
+(datum: Datum) => number;
+```
 
 #${prefix} shadowBlur(number)
 

docs/assets/option/zh/graphic/fill-style.md

@@ -10,9 +10,13 @@
 
 {{ use: graphic-gradient }}
 
-#${prefix} fillOpacity(number)
+#${prefix} fillOpacity(number|function)
 
-填充透明度。
+填充透明度。支持使用回调函数动态设置，定义如下：
+
+```ts
+(datum: Datum) => number;
+```
 
 #${prefix} shadowBlur(number)