WebView: note deprecation of onscreen zoom controls.

Inform developers that having onscreen zoom controls is deprecated and
that it's therefore not recommended to enable them in WebView, with
reference to ZoomButtonsController (which is what WebView uses to
implement them).

Bug: 141732094
Test: make ds-docs
Change-Id: I134551b87d3a93072e28aef56667507214b3e9c4