update tutorial for keyed each blocks

site/content/tutorial/04-logic/05-keyed-each-blocks/text.md

@@ -15,3 +15,5 @@ To do that, we specify a unique identifier for the `each` block:
 ```
 
 The `(thing.id)` tells Svelte how to figure out what changed.
+
+> You can use any object as the key, as Svelte uses a `Map` internally — in other words you could do `(thing)` instead of `(thing.id)`. Using a string or number is generally safer, however, since it means identity persists without referential equality, for example when updating with fresh data from an API server.