flesh out in/out transition tutorial chapter (#2792)

Author: Rich-Harris

site/content/tutorial/10-transitions/03-in-and-out/text.md

@@ -2,7 +2,13 @@
 title: In and out
 ---
 
-Instead of the `transition` directive, an element can have an `in` or an `out` directive, or both together:
+Instead of the `transition` directive, an element can have an `in` or an `out` directive, or both together. Import `fade` alongside `fly`...
+
+```js
+import { fade, fly } from 'svelte/transition';
+```
+
+...then replace the `transition` directive with separate `in` and `out` directives:
 
 ```html
 <p in:fly="{{ y: 200, duration: 2000 }}" out:fade>