docs(scroll-dispatcher): add note about callback being run outside the NgZone

[crisbeto]

Since the scrolled events are being run outside the NgZone, it might be confusing if people decide to update their view as a result of a scroll event. These changes add a note on the method docs to avoid some of the confusion.

Relates to #8279.

[jelbourn]

LGTM

[jelbourn]

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

[jelbourn]

Whoops, commented on the wrong thing

[philip-firstorder]

This saved me, thank you very much for commenting this!

For anyone out there, if you use the ScrollDispatcher to add rows to the infinite Mat-Table then you have to do like this:

import { NgZone } from '@angular/core';
import { MatTable } from '@angular/material';

@ViewChild('table') table: MatTable<Element>;

constructor(private ngZone: NgZone) {
 this.scrollDispatcher.scrolled()  
 .pipe(takeUntil(this.ngDestroy$))
 .subscribe((scrollable: CdkScrollable) => {
   const bottom = scrollable ? scrollable.measureScrollOffset('bottom') : null;
   if (bottom <= 0) { // If the user has scrolled to the bottom
     this.ngZone.run(() => {
       this.nextPage();; // Run this in `NgZone.run`, or your table will not renderRows
     });
   }
 });
}

this.nextPage()  {
 this.dataSource = data.length ? data : undefined;
 this.table.renderRows(); // This renders rows only if you used `NgZone.run` above 
}

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