docs: clarify 'each block value' means the iterated array

[okxint]

Closes #2179

Summary

The opening sentence of the keyed each blocks tutorial said:

By default, updating the value of an each block will add or remove DOM nodes at the end of the block if the size changes, and update the remaining DOM.

"The value of an each block" is ambiguous — a reader may reasonably wonder whether "value" means the array being iterated over, a per-item value, or something else. Issue #2179 points out that the phrase caused confusion because it reads as if some reactive derived value is changing, not the iterable array itself.

This PR rephrases the sentence to name the subject explicitly:

By default, when the array that an each block iterates over changes, Svelte adds or removes DOM nodes at the end of the block if the length changes, and updates the remaining DOM nodes in-place.

Test plan

Documentation-only change — no code modified.

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