Fix typo and add comment

Author: willshowell

src/cdk/overlay/keyboard/overlay-keyboard-dispatcher.ts

@@ -51,7 +51,7 @@ export class OverlayKeyboardDispatcher implements OnDestroy {
   }
 
   /**
-   * Subscribe to all key events that land on the body and dipatch those
+   * Subscribe to all key events that land on the body and dispatch those
    * events to the appropriate overlay.
    */
   private _dispatchKeyboardEvents(): void {
@@ -76,6 +76,7 @@ export class OverlayKeyboardDispatcher implements OnDestroy {
     const targetedOverlay = this._attachedOverlays.find(overlay =>
         overlay.overlayElement.contains(event.target as HTMLElement));
 
+    // Use that overlay if it exists, otherwise choose the most recently attached one
     return targetedOverlay ? targetedOverlay : overlays[overlays.length - 1];
   }