Responsive printing docs (chartjs#5167)

docs/general/responsive.md

@@ -33,3 +33,15 @@ The chart can also be programmatically resized by modifying the container size:
 ```javascript
 chart.canvas.parentNode.style.height = '128px';
 ```
+
+## Printing Resizeable Charts
+
+CSS media queries allow changing styles when printing a page. The CSS applied from these media queries may cause charts to need to resize. However, the resize won't happen automatically. To support resizing charts when printing, one needs to hook the [onbeforeprint](https://developer.mozilla.org/en-US/docs/Web/API/WindowEventHandlers/onbeforeprint) event and manually trigger resizing of each chart.
+
+```javascript
+function beforePrintHandler () {
+  for (var id in Chart.instances) {
+    Chart.instances[id].resize()
+  }
+}
+```