add options enable_device_pixel_ratio in doc

docs/en/2.options.md

@@ -122,6 +122,10 @@ These options are described in more detail below.
 > * `canvas` - Draw the lines on the canvas [default]
 > * `svg` - Using SVG to draw lines, when there are a lot of nodes and a huge area in the mind map, using this mode can bring significant performance improvements
 
+**view.enable_device_pixel_ratio** : (bool) draw HD mindmap according to the device pixel ratio.
+
+> This parameter is only used to improve the mindmap resolution when `view.engine=canvas`. It does not need to be set when the engine is `svg`.
+
 **view.hmargin** : (number) Minimum horizontal distance (in pixels) of the mindmap from the container frame  
 **view.vmargin** : (number) Minimum vertical distance (in pixels) of the mindmap from the outer container frame
 

docs/zh/2.options.md

@@ -126,6 +126,10 @@ options = {
 > * `canvas` - 把线条绘制在 canvas 上 [默认值]
 > * `svg` - 使用 svg 绘制线条，当思维导图的节点很多，面积巨大的时候，使用该模式能带来显著的性能提升
 
+**view.enable_device_pixel_ratio** : (bool) 按设备像素比例绘制高清思维导图.
+
+> 此参数仅用于改善 canvas 的绘制效果，当绘制引擎设置为 svg 时则无须设置。
+
 **view.hmargin** : (number) 思维导图距容器外框的最小水平距离（像素）  
 **view.vmargin** : (number) 思维导图距容器外框的最小垂直距离（像素）