docs: clarify reactivity in composables directory (#23731)

docs/2.guide/2.directory-structure/1.composables.md

@@ -38,6 +38,10 @@ const foo = useFoo()
 </template>
 ```
 
+::alert{type=info}
+The `composables/` directory in Nuxt does not provide any additional reactivity capabilities to your code. Instead, any reactivity within composables is achieved using Vue's Composition API mechanisms, such as ref and reactive. Note that reactive code is also not limited to the boundaries of the `composables/` directory. You are free to employ reactivity features wherever they're needed in your application.
+::
+
 :read-more{to="/docs/guide/concepts/auto-imports"}
 
 :link-example{to="/docs/examples/features/auto-imports"}