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 would love to see some best practice resolutions mentioned here for those densities. They're very nebulous otherwise, and the best I've found so far is to look at the resolutions of the images provided in the sample app.