Specify which types are allowed as values for 'key' of 'v-for'

Author: gsmnv

src/v2/guide/list.md

@@ -233,6 +233,8 @@ It is recommended to provide a `key` with `v-for` whenever possible, unless the
 
 Since it's a generic mechanism for Vue to identify nodes, the `key` also has other uses that are not specifically tied to `v-for`, as we will see later in the guide.
 
+<p class="tip">Avoid using non-primitive values as key, use string/number value instead.</p>
+
 ## Array Change Detection
 
 ### Mutation Methods