Mention “slots” (#404)

content/docs/composition-vs-inheritance.md

@@ -79,7 +79,7 @@ function App() {
 
 [Try it on CodePen.](https://codepen.io/gaearon/pen/gwZOJp?editors=0010)
 
-React elements like `<Contacts />` and `<Chat />` are just objects, so you can pass them as props like any other data.
+React elements like `<Contacts />` and `<Chat />` are just objects, so you can pass them as props like any other data. This approach may remind you of "slots" in other libraries but there are no limitations on what you can pass as props in React.
 
 ## Specialization