docs: clarify bindings for `<select multiple>` (#8260)

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

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

@@ -663,7 +663,7 @@ A `<select>` value binding corresponds to the `value` property on the selected `
 
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-A `<select multiple>` element behaves similarly to a checkbox group.
+A `<select multiple>` element behaves similarly to a checkbox group. The bound variable is an array with an entry corresponding to the `value` property of each selected `<option>`.
 
 ```sv
 <select multiple bind:value={fillings}>