Add note documenting use of DOMString for execution mode

[domfarolino]

We should prominently document the decision to use a DOMString instead of an enum for https://wicg.github.io/turtledove/#dom-auctionadinterestgroup-executionmode. The decision is documented in #479 and whatwg/webidl#893, however to avoid the question being asked again (as it has been asked a couple times during review), we should add a Note: or Advisement: in the spec about this.

/cc @caraitto