Update AtRule#nodes documentation

lib/at-rule.d.ts

@@ -86,6 +86,25 @@ declare class AtRule_ extends Container {
    * ```
    */
   name: string
+  /**
+   * An array containing the layer’s children.
+   *
+   * ```js
+   * const root = postcss.parse('@layer example { a { color: black } }')
+   * const layer = root.first
+   * layer.nodes.length           //=> 1
+   * layer.nodes[0].selector      //=> 'a'
+   * ```
+   * 
+   * Can be `undefinded` if the at-rule has no body.
+   * 
+   * ```js
+   * const root = postcss.parse('@layer a, b, c;')
+   * const layer = root.first
+   * layer.nodes                  //=> undefined
+   * ```
+   */
+  nodes: Container['nodes']
   /**
    * The at-rule’s parameters, the values that follow the at-rule’s name
    * but precede any `{}` block.