CB-3571: update the docs for the splash screens

The former documentation for splash screens was for the Phonegap build system.
The pull request updates the documentation to describe the changes made in CB-3571:
https://issues.apache.org/jira/browse/CB-3571


I think that the splash screen plugin is not the alternative, since it provides additional functionality on top of this work. So I think the following message is more accurate here:
Apache Cordova also offers special splash screen plugin which could be used to programmatically display and hide a splash screen during application launch