docs: add warning about using block elements in footer config (#2341)

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

docs/reference/default-theme-footer.md

@@ -36,4 +36,8 @@ export default {
 }
 ```
 
+::: warning
+Only inline elements can be used in `message` and `copyright` as they are rendered inside a `<p>` element. If you want to add block elements, consider using [`layout-bottom`](../guide/extending-default-theme#layout-slots) slot instead.
+:::
+
 Note that footer will not be displayed when the [SideBar](./default-theme-sidebar) is visible.