fix(tabs): update doc and fix tab demo

* update tab documentation to clarify proper use and intended behavior of the
  `active` attribute on both the `tabset` and `tab` directives.
* fix the tab select buttons in the tab demo.

Closes #5575
Fixes #5551

src/tabs/docs/demo.html

@@ -7,8 +7,8 @@
 <div ng-controller="TabsDemoCtrl">
   <p>Select a tab by setting active binding to true:</p>
   <p>
-    <button type="button" class="btn btn-default btn-sm" ng-click="scope.active = 1">Select second tab</button>
-    <button type="button" class="btn btn-default btn-sm" ng-click="scope.active = 2">Select third tab</button>
+    <button type="button" class="btn btn-default btn-sm" ng-click="active = 1">Select second tab</button>
+    <button type="button" class="btn btn-default btn-sm" ng-click="active = 2">Select third tab</button>
   </p>
   <p>
     <button type="button" class="btn btn-default btn-sm" ng-click="tabs[1].disabled = ! tabs[1].disabled">Enable / Disable third tab</button>

src/tabs/docs/readme.md

@@ -5,7 +5,7 @@ AngularJS version of the tabs directive.
 * `active`
   <i class="glyphicon glyphicon-eye-open"></i>
   _(Default: `Index of first tab`)_ -
-  Active index of tab.
+  Active index of tab. Setting this to an existing tab index will make that tab active.
 
 * `justified`
   <small class="badge">$</small>
@@ -31,7 +31,7 @@ AngularJS version of the tabs directive.
   <small class="badge">$</small>
   <i class="glyphicon glyphicon-eye-open"></i>
   _(Default: `false`)_ -
-  Whether tab is currently selected.
+  Whether tab is currently selected. Note that this is purely for CSS and will not physically set this tab to active.
 
 * `classes`
   <small class="badge">$</small> -