Add aspectRatio property to responsive doc (#5756)

docs/general/responsive.md

@@ -16,6 +16,7 @@ Chart.js provides a [few options](#configuration-options) to enable responsivene
 | `responsive` | `Boolean` | `true` | Resizes the chart canvas when its container does ([important note...](#important-note)).
 | `responsiveAnimationDuration` | `Number` | `0` | Duration in milliseconds it takes to animate to new size after a resize event.
 | `maintainAspectRatio` | `Boolean` | `true` | Maintain the original canvas aspect ratio `(width / height)` when resizing.
+| `aspectRatio` | `Number` | `2` | Canvas aspect ratio (i.e. `width / height`, a value of 1 representing a square canvas). Note that this option is ignored if the height is explicitly defined either as attribute or via the style.
 | `onResize` | `Function` | `null` | Called when a resize occurs. Gets passed two arguments: the chart instance and the new size.
 
 ## Important Note