Fix documentation for tabs disabled attribute.

The docs say use the disabled attribute but it's actually ng-disabled.

Closes angular#3756

src/components/tabs/js/tabDirective.js

@@ -20,7 +20,7 @@
  * be initiated via data binding changes, programmatic invocation, or user gestures.
  *
  * @param {string=} label Optional attribute to specify a simple string as the tab label
- * @param {boolean=} ng-disabled If present, disabled tab selection.
+ * @param {boolean=} ng-disabled If present and expression evaluates to truthy, disabled tab selection.
  * @param {expression=} md-on-deselect Expression to be evaluated after the tab has been de-selected.
  * @param {expression=} md-on-select Expression to be evaluated after the tab has been selected.
  * @param {boolean=} md-active When true, sets the active tab.  Note: There can only be one active tab at a time.
@@ -29,7 +29,7 @@
  * @usage
  *
  * <hljs lang="html">
- * <md-tab label="" disabled="" md-on-select="" md-on-deselect="" >
+ * <md-tab label="" ng-disabled md-on-select="" md-on-deselect="" >
  *   <h3>My Tab content</h3>
  * </md-tab>
  *