Add doc for MdCheckboxChange (#3514)

Add doc for MdButtonToggleChange

Author: tinayuangao

src/lib/button-toggle/button-toggle.ts

@@ -40,7 +40,9 @@ var _uniqueIdCounter = 0;
 
 /** Change event object emitted by MdButtonToggle. */
 export class MdButtonToggleChange {
+  /** The MdButtonToggle that emits the event. */
   source: MdButtonToggle;
+  /** The value assigned to the MdButtonToggle. */
   value: any;
 }
 

src/lib/checkbox/checkbox.ts

@@ -54,7 +54,9 @@ export enum TransitionCheckState {
 
 /** Change event object emitted by MdCheckbox. */
 export class MdCheckboxChange {
+  /** The source MdCheckbox of the event. */
   source: MdCheckbox;
+  /** The new `checked` value of the checkbox. */
   checked: boolean;
 }