[cdk] indicate in docs which utilities operate outside NgZone

[willshowell]

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.

[willshowell]

Agreed. Not having to hunt for the information would be good. Perhaps a prominent accent label.

A complication of a linter solution is that some utilities don't explicitly run outside of the zone, but can be called by another that does. For example, onPositionChange

https://github.com/angular/material2/blob/c13848e715c8940de3fb923e0a69ba42e67bbcb3/src/cdk/overlay/position/connected-position-strategy.ts#L74-L77

The subject is usually advanced by apply(), which is called by updatePosition() which is sometimes called within but usually outside the zone.

[saulotoledo]

@jelbourn I think one could add the "has pr" label to this issue.

[jelbourn]

The PR here was merged; it added a note to the API docs for ScrollDispatcher, but not for ViewportRuler