docs: clarify signals effect import source

Author: angular-bootstrap

adev/src/content/guide/signals/effect.md

@@ -8,6 +8,8 @@ effect(() => {
 });
 ```
 
+NOTE: `effect`, `signal`, and `computed` are exported from `@angular/core`.
+
 Effects always run **at least once.** When an effect runs, it tracks any signal value reads. Whenever any of these signal values change, the effect runs again. Similar to computed signals, effects keep track of their dependencies dynamically, and only track signals which were read in the most recent execution.
 
 Effects always execute **asynchronously**, during the change detection process.