canvas: 增加高清处理逻辑，API 暂时指向 mdn

docs/component/canvas.md

@@ -15,3 +15,30 @@
 <!-- UTSCOMJSON.canvas.example -->
 
 <!-- UTSCOMJSON.canvas.reference -->
+
+## API
+
+和 W3C 规范保持一致 [https://developer.mozilla.org/zh-CN/docs/Web/HTML/Element/canvas](https://developer.mozilla.org/zh-CN/docs/Web/HTML/Element/canvas)
+
+**注意事项**@hidpi
+
+uni-app 1 canvas 组件内部默认处理了高清逻辑，uni-app x 和 W3C 规范一致，需要开发者根据自己的场景手动处理
+
+```html
+<template>
+  <canvas id="canvas"></canvas>
+</template>
+<script setup>
+// 获取 canvas element
+const canvas = uni.getElementById("canvas") as UniCanvasElement
+const context = canvas.getContext("2d")!;
+
+// 处理高清屏逻辑
+const dpr = uni.getSystemInfoSync().pixelRatio;
+canvas.width = canvas.offsetWidth * dpr;
+canvas.height = canvas.offsetHeight * dpr;
+context.scale(dpr, dpr); // 仅需调用一次，当调用 reset 方法后需要再次 scale
+
+// 省略绘制代码，和 w3c 规范保持一致
+</script>
+```