docs: each block and keyed each block doc improvements

[repst]

This is an attempt to clarify each block and keyed each block documentation. Not sure that I got it just right but hopefully adds clarity.

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[svelte-docs-bot]

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[Rich-Harris]

Thank you but I'm afraid this doesn't seem like an improvement. 'change in state is based on an item's index in the iterated sequence' is very confusing (I'm not sure what it means), 'each iterated item' is inaccurate (the list is the thing being iterated), and 'when modifying the length of an array' is the one time that keys aren't useful.

If the word 'diff' is tripping people up then we can just change it to 'reorder', but the rest should stay as-is.