docs: document event delegation and effect deferral

[Rich-Harris]

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[AdrianGonz97]

flushSync should probably be added to the docs/imports section of the preview docs as well 🙂

[trueadm]

Yeah we should add a section on flushSync and await tick().

[dummdidumm]

tick is already documented on the main site so we don't need it here - PR's good to go from my side