#each documentation should be more helpful and direct about keys

[zzolo]

The current documentation for {#each} block only mentions keyed each blocks as a note. To me, this is a serious problem, since not using a key in many cases causes broken/unexpected results when the state/data is updated.

I think the following should happen:

• The keyed documentation should be included in the #each block documentation
• There should be clear documentation about how not using a key can lead to unexpected behavior, and is basically necessary for updating data
• (optional) The prime example for the documentation should involve keys, and the later example can talk about how not using a key can be more performant but may lead to unexpected results.

References:

• React has similar behavior: https://reactjs.org/docs/lists-and-keys.html and https://reactjs.org/docs/reconciliation.html#recursing-on-children