fix: clarify the definition of dataZoom.rangeMode.

src/component/dataZoom/DataZoomModel.js

@@ -111,7 +111,21 @@ var DataZoomModel = echarts.extendComponentModel({
         this._autoThrottle = true;
 
         /**
-         * 'percent' or 'value'
+         * It is `[rangeModeForMin, rangeModeForMax]`.
+         * The optional values for `rangeMode`:
+         * + `'value'` mode: the axis extent will always be determined by
+         *     `dataZoom.startValue` and `dataZoom.endValue`, despite
+         *     how data like and how `axis.min` and `axis.max` are.
+         * + `'percent'` mode: `100` represents 100% of the `[dMin, dMax]`,
+         *     where `dMin` is `axis.min` if `axis.min` specified, otherwise `data.extent[0]`,
+         *     and `dMax` is `axis.max` if `axis.max` specified, otherwise `data.extent[1]`.
+         *     Axis extent will be determined by the result of the percent of `[dMin, dMax]`.
+         *
+         * For example, when users are using dynamic data (update data periodically via `setOption`),
+         * if in `'value`' mode, the window will be kept in a fixed value range despite how
+         * data are appended, while if in `'percent'` mode, whe window range will be changed alone with
+         * the appended data (suppose `axis.min` and `axis.max` are not specified).
+         *
          * @private
          */
         this._rangePropMode = ['percent', 'percent'];