[cdk] indicate in docs which utilities operate outside NgZone

Bug, feature request, or proposal:

Docs request

Current behavior

It takes a little extra digging in the source to understand ScrollDispatcher#scrolled and ViewportRuler#change emit outside the zone.

Expected behavior

A note in the jsdoc of these properties and methods.

[josephperrott]

This definitely seems like something that could be good to have in docs. Though it will definitely take some designing as to how to surface it, as well as seeing if we could use something like linting to be sure that we always note ngZone usage. Worse than not documenting it, imo would be to have it inconsistently documented.

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