make mixin base classes docs private (#5301)

Author: willshowell

src/lib/tabs/tab-group.ts

@@ -37,6 +37,7 @@ export class MdTabChangeEvent {
 export type MdTabHeaderPosition = 'above' | 'below';
 
 // Boilerplate for applying mixins to MdTabGroup.
+/** @docs-private */
 export class MdTabGroupBase {
   constructor(public _renderer: Renderer2, public _elementRef: ElementRef) {}
 }

src/lib/tabs/tab-nav-bar/tab-nav-bar.ts

@@ -33,6 +33,7 @@ import 'rxjs/add/observable/merge';
 import {Subject} from 'rxjs/Subject';
 
 // Boilerplate for applying mixins to MdTabNav.
+/** @docs-private */
 export class MdTabNavBase {
   constructor(public _renderer: Renderer2, public _elementRef: ElementRef) {}
 }