docs(LifecycleHooks): correct ngDoCheck description (#8807)

The current behavior is for ngDoCheck to supplement, not override, the default change detector. OnChanges will still be called when DoCheck is implemented (fixes #7307).

Author: zbjornson

modules/@angular/core/src/metadata/lifecycle_hooks.ts

@@ -130,22 +130,19 @@ export abstract class OnChanges { abstract ngOnChanges(changes: SimpleChanges);
 export abstract class OnInit { abstract ngOnInit(); }
 
 /**
- * Implement this interface to override the default change detection algorithm for your directive.
+ * Implement this interface to supplement the default change detection algorithm in your directive.
  *
- * `ngDoCheck` gets called to check the changes in the directives instead of the default algorithm.
+ * `ngDoCheck` gets called to check the changes in the directives in addition to the default algorithm.
  *
  * The default change detection algorithm looks for differences by comparing bound-property values
- * by reference across change detection runs. When `DoCheck` is implemented, the default algorithm
- * is disabled and `ngDoCheck` is responsible for checking for changes.
+ * by reference across change detection runs.
  *
- * Implementing this interface allows improving performance by using insights about the component,
- * its implementation and data types of its properties.
+ * Note that a directive typically should not use both `DoCheck` and {@link OnChanges} to respond to
+ * changes on the same input. `ngOnChanges` will continue to be called when the default change detector
+ * detects changes, so it is usually unnecessary to respond to changes on the same input in both hooks.
+ * Reaction to the changes have to be handled from within the `ngDoCheck` callback.
  *
- * Note that a directive should not implement both `DoCheck` and {@link OnChanges} at the same time.
- * `ngOnChanges` would not be called when a directive implements `DoCheck`. Reaction to the changes
- * have to be handled from within the `ngDoCheck` callback.
- *
- * Use {@link KeyValueDiffers} and {@link IterableDiffers} to add your custom check mechanisms.
+ * You can use {@link KeyValueDiffers} and {@link IterableDiffers} to help add your custom check mechanisms.
  *
  * ### Example ([live demo](http://plnkr.co/edit/QpnIlF0CR2i5bcYbHEUJ?p=preview))
  *