Doc: Added the missing \qmlmethod statements

It is necessary to use appropriate \qmlxxx QDoc
commands to document the WebView properties and
methods, as WebView is a C++ class documented
as a QML type.

Change-Id: I6001b6f4e19553935d8e22ed7c1f99c7e06b2b80
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>