Update onPause documentation

onPause() method documentation wrongly claims it stops JS. This could
have been true for very early implementations of Webview which does
not use V8 (pre-honeycomb) however it is not the case anymore
since V8 does not provide a way to stop JS safely for a particular Webview.

Change-Id: I6363dc2ef47c463da6baa417832da750225f5cf2