docs(core): indicate OnPush applies to children (#28320)

Indicate the OnPush change detection strategy applies to all child directives.
Resolves confusion from #12480.
PR Close #28320

packages/core/src/change_detection/constants.ts

@@ -18,6 +18,7 @@ export enum ChangeDetectionStrategy {
    * Use the `CheckOnce` strategy, meaning that automatic change detection is deactivated
    * until reactivated by setting the strategy to `Default` (`CheckAlways`).
    * Change detection can still be explicitly invoked.
+   * This strategy applies to all child directives and cannot be overridden.
    */
   OnPush = 0,