docs: note default value breaking change to bind: in runes mode

[dummdidumm]

#11400 (comment)

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[FoHoOV]

I guess this can be mentioned as well (kinda the same but worth mentioning?) - My grammer sux soooo, forgive me :D:
While binding to bindable props within an each block, the inital array must have the same length (or bigger) as what the each block is iterating over. Previously svelte pushed to the array internally, but now it will use whatever is in the array.
Specifially:

<script>
      import Something from "./Something.svelte";
	
-     let boundValues = [];
+     let boundValues = $state(Array(3).fill(null)); // fill it with anything else other than undefined. 
</script>

{#each {length: 3} as _, i}
	<Something bind:value={boundValues[i]}></Something>
{/each}

Note the Array(3), is creating 3 elements, same length as what we are iterating over. For example using Array(2) will result in an error, because boundValues[2] is undefined.

---

Current changeset's focus is more around the default value propagation but this will also put a focus on the fact that we have to change the initial variable that we are binding to - which is specific to #each blocks AFAIK.

[Rich-Harris]

I think the each stuff is overkill — it's an edge case and it's not new information, it's something that's implied what's already there