Primeng: Document the lazy requirement for virtual scroll

[jdp80]

I'm submitting a ... (check one with "x")

[x] bug report => Search github for a similar issue or PR before submitting
[ ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Plunkr Case (Bug Reports)
http://plnkr.co/tYCqxKMhajxqrHVbG8EM?p=preview

Current behavior
The table updates but the offset top callback does not run, meaning that the changes to data are not visible within the visible component area

Expected behavior
Before 4.3.0, virtualScroll did not require lazy loading enabled to work.

Minimal reproduction of the problem with instructions
Create a large dataset with a small rows count, observe that the callback of DataTable ScrollableView.onVirtualScroll does not get called unless DataTable.handleChange runs.
With default immutable, this can only happen from the DataTable value setter

What is the motivation / use case for changing the behavior?
Documentation does not specify this as a mandatory combination

• Angular version:
• PrimeNG version:
  Confirmed in
  Angular 4.4.4 + PrimeNG 4.3.0
  Angular 5.x + PrimeNG 5.0.0

[jdp80]

This also means that the built-in sorting/filtering functions no longer function and need to be duplicated in client code

[cagataycivici]

Will be documented.

[nate-knight]

Is lazyloading a requirement for virtual scrolling going forward or is this going to be fixed?

[cagataycivici]

Tracked at;

https://github.com/primefaces/primeng/issues/7419