Add ScalaDoc to raw.Html.tabIndex

Author: japgolly

src/main/scala/org/scalajs/dom/raw/Html.scala

@@ -3624,9 +3624,28 @@ abstract class HTMLElement extends Element {
  * MDN
  */
  var contentEditable: String = js.native
+
  /**
  * Gets/sets the tab order of the current element.
  *
+ * The '''tabindex''' global attribute is an integer indicating if the element can take input
+ * focus (is ''focusable''), if it should participate to sequential keyboard navigation, and if
+ * so, at what position. It can take several values:
+ *
+ * - a ''negative value'' means that the element should be focusable, but should not be
+ * reachable via sequential keyboard navigation;
+ * 
+ * - '''0''' means that the element should be focusable and reachable via sequential keyboard
+ * navigation, but its relative order is defined by the platform convention;
+ * 
+ * - a ''positive value'' which means should be focusable and reachable via sequential keyboard
+ * navigation; its relative order is defined by the value of the attribute: the sequential
+ * follow the increasing number of the '''tabindex'''. If several elements share the same
+ * tabindex, their relative order follows their relative position in the document).
+ * 
+ * An element with a '''0''' value, an invalid value, or no '''tabindex''' value should be placed
+ * after elements with a positive '''tabindex''' in the sequential keyboard navigation order.
+ *
  * MDN
  */
  var tabIndex: Int = js.native