docs: Clarify when bind:group does not work (#8540)

Clarify documentation around when bind:group does and does not work. See issue #2308

Author: abirtley

site/content/docs/03-template-syntax.md

@@ -823,6 +823,8 @@ Inputs that work together can use `bind:group`.
 <input type="checkbox" bind:group={fillings} value="Guac (extra)">
 ```
 
+> `bind:group` only works if the inputs are in the same Svelte component.
+
 #### bind:this
 
 ```sv