📕 amp-script: Document how and when worker-doc releases are included

[sebastianbenz]

Context: I'm waiting for this worker-dom PR to make it into amp-script. I don't know how to track the release.

It'd be great to have two things:

1. Add an FAQ entry in the amp-script documentation explaining how long it usually takes for a worker-doc feature to end in amp-script.
2. Document the currently used worker-dom version (ideally with a link to version specific API docs).

//cc @kristoferbaxter @choumx

[sebastianbenz]

See also: ampproject/worker-dom#658

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