Clarify that IDBTransaction.onerror fires when errors bubble up from requests

[taymonbeal]

@inexorabletash: "The spec is correct for implementers, but it could use some non-normative webdev-facing text about the event handlers."

[inexorabletash]

This question comes up a lot.

https://w3c.github.io/IndexedDB/#dom-idbtransaction-onerror says:

To determine if a transaction has completed successfully, listen to the transaction's complete event rather than the success event of a particular request, because the transaction can still fail after the success event fires.

...but this could be extended with more details..