Improve bar documentation for field "seriesField". (#555)

* Improve bar documentation for field "seriesField".

Explain how to avoid issues with stacked bars order.
Example added.

* Fix: improve text understanding.

Use word "xField" instead of "value" to avoid confusion.

docs/.g2plot-plot-api/bar.md

@@ -80,6 +80,42 @@ The name of the data field corresponding to the graph in the y direction, usuall
 
 Grouping field. It is the same meaning as groupField、colorField in Grouped Bar, and the same as stackField、colorField in Stacked Bar.
 
+To avoid issues related to stacked bars order, make sure each `yField` gets a bar defined for each possible value of `seriesField`, even if `xField` is 0.
+
+Example：
+
+```ts
+config = {
+    data: [
+        {
+            x: 0,
+            y: "bar",
+            color: "red"
+        },
+        {
+            x: 40,
+            y: "bar",
+            color: "yellow"
+        },
+        {
+            x: 2,
+            y: "foo",
+            color: "red"
+        },
+        {
+            x: 40,
+            y: "foo",
+            color: "yellow"
+        }
+    ].reverse(),
+    isStack: true,
+    xField: "x",
+    yField: "y",
+    seriesField: "color",
+    color: _ref => _ref.color
+};
+```
+
 #### groupField
 
 <description>**optional** *string*</description>