Clarify WebSettings comments related to zooming.

1. It wasn't clear that SupportZoom doesn't affect WebView's
   zoomIn / zoomOut methods.

2. It was easy to misread that to enable "pinch to zoom" one
   needs to hide zoom controls.

Bug: 5461416

Change-Id: Ic8afd22ae0f36e53b9f4724e95ac55c766f58af5